ieee802154: mrf24j40: Add support for MRF24J40MC

The MRF24J40MC module has an external amplifier which should be
enabled. The TX power has to be lowered to meet FCC regs.

Signed-off-by: Simon Vincent <simon.vincent@xsilon.com>
Acked-by: Alan Ott <alan@signal11.us>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>