drivers: memc: stm32: FMC NOR/PSRAM refactor

Simplifies the driver code:
* Use existing FMC_NORSRAM_DEVICE and FMC_NORSRAM_EXTENDED_DEVICE
  defines. No need to keep references to them in the driver's config.
* Refine initialization loop.

Signed-off-by: Georgij Černyšiov <geo.cgv@gmail.com>