MIPS: BCM47xx: Move SPROM driver to drivers/firmware/

MIPS: BCM47xx: Move SPROM driver to drivers/firmware/

Broadcom ARM home routers store SPROM content in NVRAM just like MIPS
ones. To share SPROM code we need to move it out of arch/mips/ to some
common place. We already have bcm47xx_nvram in firmware path and SPROM
should fit there as well.
This driver is responsible for parsing SoC configuration data into a
struct shared between ssb and bcma buses.
This was tested with BCM4706 & BCM5357C0 (BCM47XX) and BCM4708A0
(ARCH_BCM_5301X).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12210/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>