gpio: raspberrypi-exp: explain Kconfig dependency

Commit 7ed915059c3001 (gpio: raspberrypi-ext: fix firmware dependency)
fixed the Kconfig dependency to ensure that gpio-raspberrypi-exp is not
built-in when the firmware is a module. But the Kconfig syntax for doing
so is cryptic. Add a comment to make it a little easier.

Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/Kconfig

@@ -126,6 +126,8 @@ config GPIO_RASPBERRYPI_EXP
 	tristate "Raspberry Pi 3 GPIO Expander"
 	default RASPBERRYPI_FIRMWARE
 	depends on OF_GPIO
+	# Make sure not 'y' when RASPBERRYPI_FIRMWARE is 'm'. This can only
+	# happen when COMPILE_TEST=y, hence the added !RASPBERRYPI_FIRMWARE.
 	depends on (ARCH_BCM2835 && RASPBERRYPI_FIRMWARE) || (COMPILE_TEST && !RASPBERRYPI_FIRMWARE)
 	help
 	  Turn on GPIO support for the expander on Raspberry Pi 3 boards, using