gpio: raspberrypi-exp: explain Kconfig dependency

Commit 7ed91505 (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>