MAINTAINERS: add entry for the Broadcom Northstar Plus SoCs

Modify the iProc MAINTAINER to cover the Nortstar Plus SoCs, and add me
to the list of people responsible for it.  Since iProc is a family of
SoCs that includes more than just Cygnus (i.e., Northstar (BCM5301X),
Northstar Plus, Northstar 2, and a number of other platforms), remove it
from the title to avoid any future confusion.

Signed-off-by: Jon Mason <jonmason@broadcom.com>
Acked-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>