mtd: bcm47xxpart: limit scanned flash area on BCM47XX (MIPS) only

We allowed using bcm47xxpart on BCM5301X arch with commit:
9e3afa5f ("mtd: bcm47xxpart: allow enabling on ARCH_BCM_5301X")

BCM5301X devices may contain some partitions in higher memory, e.g.
Netgear R8000 has board_data at 0x2600000. To detect them we should
use size limit on MIPS only.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>