MAINTAINERS: Add section for MRF24J40 IEEE 802.15.4 radio driver

Alan is the original author of the driver. This change was discussed
with the 802.15.4 subsystem maintainer, Alexander Aring.

Signed-off-by: Alan Ott <alan@signal11.us>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>