MAINTAINERS: Entry for Cygnus/iproc arm architecture

Acked-by: Jonathan Richardson <jonathar@broadcom.com>
Signed-off-by: Scott Branden <sbranden@broadcom.com>
ARM: mach-bcm: Consolidate currently supported IPROC SoCs

Move ARCH_BCM_5301X subarch under ARCH_IPROC architecture.
Additional IPROC chipsets that share a lot of commonality should be
added under ARCH_IPROC as well.