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.

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