[ip17x] [etherswitch] fdt away and mii hang workaround on ip17x

Add workaround mii access because of rt1310 is hang up on etherswitch mii poll.
And FDT away on arm platform.

Tested:

• wzr2-g300n

Submitted by: Hiroki Mori <yamori813@yahoo.co.jp>
Reviewed by: mizhka
Differential Revision: https://reviews.freebsd.org/D10295