drivers/flash/soc_flash_sam: integrate WP service into write/erase

Flash write protection services were integrated into erase and write
procedures. This is step required for fixing following issue:
Multi-threading flash access is not supported by
flash_write_protection_set().

Write procedure was fixed so it doesn't lock the driver after
failure anymore.

flash_write_protection_set() will be deprecated.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>