MIPS: BCM47xx: Support on-SoC bus in SPROM reading function

MIPS: BCM47xx: Support on-SoC bus in SPROM reading function

To support (extract) SPROM on Broadcom ARM devices we should separate
SPROM code and make it a separated module. We won't want to export
bcm47xx_fill_sprom symbol so we should support SoC SPROM in the standard
fallback function and then modify ssb to use it.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11355/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>