Make the GPIO part of the driver optional, under a boolean config
option. Move the dependency to GPIOLIB and OF and the selection of
GPIOLIB_IRQCHIP to this new option.

This makes the turris-omnia-mcu driver available for compilation even if
GPIOLIB or OF are disabled.

Fixes: ed46f1f7 ("platform: cznic: turris-omnia-mcu: fix Kconfig dependencies")
Signed-off-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/20240719085756.30598-5-kabel@kernel.orgSigned-off-by: Arnd Bergmann <arnd@arndb.de>