drivers: gpio: Add support for GPIOs over Moxtet bus

This adds support for interpreting the input and output bits of one
device on Moxtet bus as GPIOs.
This is needed for example by the SFP cage module of Turris Mox.

Link: https://lore.kernel.org/r/20190812161118.21476-5-marek.behun@nic.cz
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

drivers/gpio/Kconfig

@@ -1445,6 +1445,15 @@ config GPIO_XRA1403
 	help
 	  GPIO driver for EXAR XRA1403 16-bit SPI-based GPIO expander.
 
+config GPIO_MOXTET
+	tristate "Turris Mox Moxtet bus GPIO expander"
+	depends on MOXTET
+	help
+	  Say yes here if you are building for the Turris Mox router.
+	  This is the driver needed for configuring the GPIOs via the Moxtet
+	  bus. For example the Mox module with SFP cage needs this driver
+	  so that phylink can use corresponding GPIOs.
+
 endmenu
 
 menu "USB GPIO expanders"