Add some initial PCIe bridge support for the AR724x chipsets.

This is reported to work on the AR7240 based Ubiquiti Rocket M5
but I haven't tested it on that hardware. I also don't yet have
it fully working on the AR7242 based development board here;
probe/attach functions but the register space resource looks like
the endian-ness is wrong (0x10000000 instead of 0x00001000).o

Further digging will be required.

Submitted by: Luiz Otavio O Souza