Add proper PCIe init for MT7628/MT7688 SoCs

PCIe PHY needs different initialization on MT7628/MT7688 SoCs than it does
on MT7620.
However, LEDE (and OpenWRT) dts files have the PCIe node for MT7628/MT7688
as compatible with mt7620-pci.
We already can handle this properly in our driver, so we just need to add
compat strings to fbsd-mt7628an.dtsi and the PCIe driver.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D6395