iio: proximity: Add SX9324 support

Semtech SAR sensor SX9324 is an evolution of the SX9310:
It has 4 phases that can be configure to capture and process data
from any of 3 CS pins and provide independent detection:
proximity, table proximity or body proximity.

Gather antenna data:
echo sx9324-dev3 > trigger/current_trigger
echo 1 > scan_elements/in_proximity0_en
echo 1 > buffer/enable
od -v -An --endian=big -t d2 -w2 /dev/iio\:device3
(at 10Hz, the default).

Trigger events:
Setting:
thresh_falling_period: 2 (events)
thresh_rising_period: 2 (events)
in_proximity0_thresh_either_value: 300
in_proximity0_thresh_either_hysteresis: 72

using iio_event_monitor /dev/iio\:deviceX, approaching my hand to the
antenna pad, I see:
...
Event: time: 1634763907532035297, type: proximity, channel: 0, evtype:
thresh, direction: falling
Event: time: 1634763910138104640, type: proximity, channel: 0, evtype:
thresh, direction: rising
...

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>

Documentation/ABI/testing/sysfs-bus-iio-sx9324

@@ -0,0 +1,28 @@
+What:		/sys/bus/iio/devices/iio:deviceX/in_proximity<id>_setup
+Date:		November 2021
+KernelVersion:	5.17
+Contact:	Gwendal Grignou <gwendal@chromium.org>
+Description:
+		SX9324 has 3 inputs, CS0, CS1 and CS2. Hardware layout
+		defines if the input is
+		+ not connected (HZ),
+		+ grounded (GD),
+		+ connected to an antenna where it can act as a base
+		  (DS - data shield), or measured input (MI).
+
+		The sensor rotates measurement across 4 phases
+		(PH0, PH1, PH2, PH3), where the inputs are configured
+		and then measured.
+
+		By default,  during the first phase, [PH0], CS0 is measured,
+		while CS1 and CS2 are used as shields.
+		`cat in_proximity0_setup` returns "MI,DS,DS".
+		[PH1], CS1 is measured, CS0 and CS2 are shield:
+		`cat in_proximity1_setup` returns "DS,MI,DS".
+		[PH2], CS2 is measured, CS0 and CS1 are shield:
+		`cat in_proximity1_setup` returns "DS,DS,MI".
+		[PH3], CS1 and CS2 are measured (combo mode):
+		`cat in_proximity1_setup` returns "DS,MI,MI".
+
+		Note, these are the chip default. Hardware layout will most
+		likely dictate different output. The entry is read-only.

drivers/iio/proximity/Kconfig

@@ -129,6 +129,20 @@ config SX9310
 	  To compile this driver as a module, choose M here: the
 	  module will be called sx9310.
 
+config SX9324
+	tristate "SX9324 Semtech proximity sensor"
+	select IIO_BUFFER
+	select IIO_TRIGGERED_BUFFER
+	select REGMAP_I2C
+	select SX_COMMON
+	depends on I2C
+	help
+	  Say Y here to build a driver for Semtech's SX9324
+	  proximity/button sensor.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called sx9324.
+
 config SX9500
 	tristate "SX9500 Semtech proximity sensor"
 	select IIO_BUFFER

drivers/iio/proximity/Makefile

@@ -14,6 +14,7 @@ obj-$(CONFIG_RFD77402)		+= rfd77402.o
 obj-$(CONFIG_SRF04)		+= srf04.o
 obj-$(CONFIG_SRF08)		+= srf08.o
 obj-$(CONFIG_SX9310)		+= sx9310.o
+obj-$(CONFIG_SX9324)		+= sx9324.o
 obj-$(CONFIG_SX_COMMON) 	+= sx_common.o
 obj-$(CONFIG_SX9500)		+= sx9500.o
 obj-$(CONFIG_VCNL3020)		+= vcnl3020.o