Introduce Mediatek/Ralink PCIe support

This revision introduces PCIe support for the relevant Mediatek/Ralink
SoCs.
Currently the PCIe support is not converted to INTRNG, this may be a
task for the future.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5886