gpio: add support for FTDI's MPSSE as GPIO

FTDI FT2232H is a USB to GPIO chip. Sealevel produces some devices
with this chip. FT2232H presents itself as a composite device with two
interfaces (each is an "MPSSE"). Each MPSSE has two banks (high and low)
of 8 GPIO each. I believe some MPSSE's have only one bank, but I don't
know how to identify them (I don't have any for testing) and as a result
are unsupported for the time being.

Additionally, this driver provides software polling-based interrupts for
edge detection. For the Sealevel device I have to test with, this works
well because there is hardware debouncing. From talking to Sealevel's
people, this is their preferred way to do edge detection.

Signed-off-by: Mary Strodl <mstrodl@csh.rit.edu>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20241009131131.1618329-1-mstrodl@csh.rit.edu
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/Kconfig

@@ -1853,6 +1853,13 @@ config GPIO_VIPERBOARD
 	  River Tech's viperboard.h for detailed meaning
 	  of the module parameters.
 
+config GPIO_MPSSE
+	tristate "FTDI MPSSE GPIO support"
+	select GPIOLIB_IRQCHIP
+	help
+	  GPIO driver for FTDI's MPSSE interface. These can do input and
+	  output. Each MPSSE provides 16 IO pins.
+
 endmenu
 
 menu "Virtual GPIO drivers"

drivers/gpio/Makefile

@@ -115,6 +115,7 @@ obj-$(CONFIG_GPIO_MOCKUP)		+= gpio-mockup.o
 obj-$(CONFIG_GPIO_MOXTET)		+= gpio-moxtet.o
 obj-$(CONFIG_GPIO_MPC5200)		+= gpio-mpc5200.o
 obj-$(CONFIG_GPIO_MPC8XXX)		+= gpio-mpc8xxx.o
+obj-$(CONFIG_GPIO_MPSSE)		+= gpio-mpsse.o
 obj-$(CONFIG_GPIO_MSC313)		+= gpio-msc313.o
 obj-$(CONFIG_GPIO_MT7621)		+= gpio-mt7621.o
 obj-$(CONFIG_GPIO_MVEBU)		+= gpio-mvebu.o