if_awg: Support new emac bindings

Highlights of the new bindings:

• ahb clock is specified as 'stmmaceth'
• The PHY to be used is now specified as phy-handle
• We must now check the parent of the node phy-handle points to in order to

discover if we're using internal PHY.

• The ephy clk/reset will be specified on the PHY node, not the emac node.

Care has been taken to ensure that we remain compatible with the older
bindings that we were previously using.

Tested on: Pine64 (A64, old bindings)
Tested on: Pine64-LTS (A64, new bindings) [manu]
Tested on: OrangePi-One (H3, internal PHY) [manu]
Tested on: NanoPi M1 Plus (H3, external PHY) [manu]
Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D13777