drivers: stm32-fmc: enable MEMC_STM32 default based on dt

Default the option MEMC_STM32 to "y" when the device node is defined and
enabled, so that the driver is selected automatically when the board
supports it and MEMC is enabled.

Remove the default conditional on serise as it's redundant with the one
in soc/arm/st_stm32/common/Kconfig.defconfig.series.

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>