MIPS: BCM47XX: Support SPROM prefixes for PCI devices

Support parsing SPROMs with prefixes defined like devpath1=pci/1/1

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/9552/


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