drivers/modem: Add generic cellular modem driver

The added cellular modem driver is a naive driver, which
shall serve as a template for implementing tailored
drivers for modems like the UBLOX-R4. It uses only
generic at commands, described in 3gpp, and protocols,
like CMUX and PPP.

If finalized, it can replace the gsm_ppp entirely in the
future. The goal of the driver however is to serve as a
bare minimum, not a production ready driver.

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>

drivers/modem/CMakeLists.txt

@@ -41,3 +41,5 @@ if (CONFIG_MODEM_SIM7080)
 	zephyr_library_include_directories(${ZEPHYR_BASE}/subsys/net/ip)
 	zephyr_library_sources(simcom-sim7080.c)
 endif()
+
+zephyr_library_sources_ifdef(CONFIG_MODEM_CELLULAR modem_cellular.c)

drivers/modem/Kconfig

@@ -192,6 +192,7 @@ source "drivers/modem/Kconfig.ublox-sara-r4"
 source "drivers/modem/Kconfig.quectel-bg9x"
 source "drivers/modem/Kconfig.wncm14a2a"
 source "drivers/modem/Kconfig.gsm"
+source "drivers/modem/Kconfig.cellular"
 
 source "drivers/modem/Kconfig.hl7800"
 source "drivers/modem/Kconfig.simcom-sim7080"

drivers/modem/Kconfig.cellular

@@ -0,0 +1,39 @@
+# Copyright (c) 2023 Bjarki Arge Andreasen
+# SPDX-License-Identifier: Apache-2.0
+
+config MODEM_CELLULAR
+	bool "Generic cellular modem support"
+	select MODEM_MODULES
+	select MODEM_PPP
+	select MODEM_CMUX
+	select MODEM_CHAT
+	select MODEM_PIPE
+	select MODEM_BACKEND_UART
+	select RING_BUFFER
+	depends on DT_HAS_QUECTEL_BG9X_ENABLED || DT_HAS_ZEPHYR_GSM_PPP_ENABLED || \
+		   DT_HAS_SIMCOM_SIM7080_ENABLED || DT_HAS_UBLOX_SARA_R4_ENABLED
+	help
+	  This driver uses the generic 3gpp AT commands, along
+	  with the standard protocols CMUX and PPP, to configure
+	  cellular modems to establish a point-to-point
+	  network connection. It is a plug-in replacement for the
+	  existing GSM_PPP driver, and a template for tailored
+	  drivers for the ublox, quectel and other modems, which
+	  include power management and more complex device specific
+	  features.
+
+if MODEM_CELLULAR
+
+config MODEM_CELLULAR_APN
+	string "APN"
+	default "internet"
+
+config MODEM_CELLULAR_USERNAME
+	string "APN username"
+	default ""
+
+config MODEM_CELLULAR_PASSWORD
+	string "APN password"
+	default ""
+
+endif