Add initial support for the QCA955x PCIe host controller.

The QCA955x looks a lot like the AR724x PCIe controller, except it
supports two root complexes. Unfortunately I only have one, so
although this code has started down the path of supporting more than
one, it's definitely not yet ready.

Tested:

• AP135 board (QCA9558 SoC), with the 11ac NIC swapped for an AR9380 PCIe NIC.

Notes:

• Yes, this driver isn't very pretty. I decided to commit what I have versus holding onto something that isn't yet finished. It is enough to bring up the above NIC and interrupt routing works, so it's a good start.

• However, yes, the DDR flush routine hooks need to be fixed up. I don't think I'm firing the right one at the moment.