Add support for bcm54213PE in brgphy.

Add support for bcm54213PE in brgphy.

This chip is used in the Rasperry Pi 4, and is supported by the if_genet
driver. Currently we use the ukphy mii driver, this patch switches over
to the brgphy mii driver instead. To support the rgmii-rxid phy mode,
which is now the default in the Linux dtb, we add support for clock
skewing.

These changes are taken from OpenBSD and NetBSD, except for the bailout
in brgphy_bcm54xx_clock_delay() in rgmii mode, which was found necessary
after testing.

Submitted by:	Robert Crowston, crowston at protomail.com
Differential Revision:	https://reviews.freebsd.org/D25251