MIPS: Loongson: Get host bridge information

Read the address of host bridge configuration space to get the vendor ID
and device ID of host bridge, and then we can distinguish various types
of host bridge such as LS7A or RS780E.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>