MIPS: BCM47XX: Move SPROM fallback code into sprom.c

This is some general cleanup as well as preparing sprom.c to become a
standalone driver. We will need this for bcm53xx ARM arch support.

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/8232/


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