Add a GPIO driver for the Arm pl061 controller

A PL061 is a simple 8 pin GPIO controller. This GPIO device is used to
signal an internal request for shutdown on some virtual machines including
Arm-based Amazon EC2 instances.

Submitted by:	Ali Saidi <alisaidi_amazon.com> (previouss version)
Reviewed by:	Ali Saidi, manu
Differential Revision:	https://reviews.freebsd.org/D24065

Author: zxombie

sys/arm64/conf/GENERIC

@@ -257,6 +257,7 @@ device		gpioregulator
 device		ls1046_gpio	# LS1046A GPIO controller
 device		mv_gpio		# Marvell GPIO controller
 device		mvebu_pinctrl	# Marvell Pinmux Controller
+device		pl061		# Arm PL061 GPIO controller
 device		rk_gpio		# RockChip GPIO Controller
 device		rk_pinctrl	# RockChip Pinmux Controller
 

sys/conf/files.arm64

@@ -305,6 +305,9 @@ dev/iicbus/sy8106a.c		optional	sy8106a fdt
 dev/iicbus/twsi/mv_twsi.c	optional	twsi fdt
 dev/iicbus/twsi/a10_twsi.c	optional	twsi fdt
 dev/iicbus/twsi/twsi.c		optional	twsi fdt
+dev/gpio/pl061.c		optional	pl061 gpio
+dev/gpio/pl061_acpi.c		optional	pl061 gpio acpi
+dev/gpio/pl061_fdt.c		optional	pl061 gpio fdt
 dev/hwpmc/hwpmc_arm64.c		optional	hwpmc
 dev/hwpmc/hwpmc_arm64_md.c	optional	hwpmc
 dev/mbox/mbox_if.m		optional	soc_brcm_bcm2837