drivers/flash: Add explict erase capability with proper values

Add explicit_erase capability and set proper value to it,
depending on type of a device.
Select FLASH_HAS_EXPLICIT_ERASE by devices that have program-erase
characteristics and FLASH_HAS_NO_EXPLICIT_ERASE, where needed.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>