platform: cznic: turris-omnia-mcu: fix Kconfig dependencies

The newly added driver causes a Kconfig warning:

WARNING: unmet direct dependencies detected for RTC_CLASS
  Depends on [n]: !S390 [=y]
  Selected by [m]:
  - TURRIS_OMNIA_MCU [=m] && CZNIC_PLATFORMS [=y] && (MACH_ARMADA_38X || COMPILE_TEST [=y]) && I2C [=m] && OF [=y] && WATCHDOG [=y]

The problem here is that it selects entire subsystems, which normal
device drivers should not do. Changes all of these to 'depends on'
instead.

Fixes: dfa556e4 ("platform: cznic: turris-omnia-mcu: Add support for MCU connected GPIOs")
Fixes: 90e700fd ("platform: cznic: turris-omnia-mcu: Add support for poweroff and wakeup")
Fixes: ab89fb5f ("platform: cznic: turris-omnia-mcu: Add support for MCU watchdog")
Fixes: 41bb142a ("platform: cznic: turris-omnia-mcu: Add support for MCU provided TRNG")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>