ath79: fortinet-fap-221-b: convert to nvmem-layout

ath79: fortinet-fap-221-b: convert to nvmem-layout

Now that MAC address parser supports the hex format (without
delimiters), use the canonical MAC address stored in U-boot partition.
Get rid of the userspace adjustments which are no longer necessary.
While at that, move the mac-base to the common part, as it is again
exactly the same in both models.

And convert ART partition too - keep that one separate, as calibration
data length differs between the models.

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>