dt-bindings: PCI: Add compatible string for Brcmstb 74[23]5 MIPs SOCs

The Broadcom STB Arm and MIPs SOCs use the same PCIe controller
HW, although the MIPs version is older.

Signed-off-by: Jim Quinlan <jim2101024@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>