mips: pci-mt7620: fix bridge register access

mips: pci-mt7620: fix bridge register access

Host bridge registers and PCI RC control registers have different
memory base. pcie_m32() is used to write the RC control registers
instead of bridge registers. This patch introduces bridge_m32()
and use it to operate bridge registers to fix the access issue.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>