MIPS: BCM47XX: Select SYS_SUPPORTS_HIGHMEM for BCM47XX_BCMA

It seems that bcm47xx can handle only 128 MiB of RAM directly. There
are few devices with 256 MiB, but Broadcom's SDK uses highmem to handle
anything above 128 MiB.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: https://patchwork.linux-mips.org/patch/7101/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>