if_awg: Use syscon prop if it exists

The emac bindings that are landing in Linux 4.15 specify a syscon property
on the emac node that point to /soc/syscon. Use this property if it's
specified, but maintain backwards compatibility with the old method.

The older method is still used for boards that we get .dtb from u-boot, such
as pine64, that did not yet have stable emac bindings.

Tested on: Banana Pi-M3 (a83t)
Tested on: Pine64 (a64)
Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D13296