MIPS: BCM47xx: Support SPROM prefixes on other platforms

MIPS: BCM47xx: Support SPROM prefixes on other platforms

BCM47XX platform has specific PCI setup because all buses share the same
domain. It's different e.g. on ARM ARCH_BCM_5301X where each PCI bus
gets its own domain (they are handled by iProc PCIe controller driver).

As we want to make SPROM driver more generic, let's add an exception for
BCM47xx. It was tested on BCM4706 (MIPS) and BCM4708A0 (ARM).

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/11969/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>