[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