ath79: dts: drop "simple-mfd" for gmacs in SoC dtsi

ath79: dts: drop "simple-mfd" for gmacs in SoC dtsi

With a proper probe deferring for ag71xx we don't need to explicitly
probe mdio1 before gmac0.
Drop all "simple-mfd" in SoC dtsi so that gmac orders can be the same
as ar71xx.
This makes eth0/eth1 order the same as those in ar71xx, which means
we don't need a migration script for this anymore and we can merge
incorrectly split gmac/mdio driver back together.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>