ath79: ar724x: make sure builtin-switch is enabled in DT

ath79: ar724x: make sure builtin-switch is enabled in DT

On ar7240/ar7241 the mdioX node with the builtin-switch is enabled
in the DTSI files, but the parent ethX node is left disabled. It
only gets enabled per device or device family, and has not been
enabled at all yet for the TP-Link WA devices with ar7240, making
the switch unavailable there.

This patch makes sure &eth0/&eth1 nodes are enabled together with
the &mdio0/&mdio1 nodes containing the builtin-switch.
For ar7240_tplink_tl-wa.dtsi, &eth0 is properly hidden again via
  compatible = "syscon", "simple-mfd";

This partially fixes FS#2887, however it seems dmesg still does
not show cable (dis)connect in dmesg for ar7240 TP-Link WA
devices.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>