[ar531x] add initial port for the AR231x/531x series of SoCs.

These are older MIPS4kc parts from Atheros. They typically ran at
sub-200MHz and have 11bg, 11a, or 11abg wifi MAC/PHYs integrated.

This port is the initial non-wifi pieces required to bring up the
chip. I'll commit the redboot and other pieces later, and then
hopefully(!) wifi support will follow.

Submitted by: Mori Hiroki <yamori813@yahoo.co.jp>
Differential Revision: https://reviews.freebsd.org/D7237