ath79: ag71xx: add support for optional mdio reset

On ar933x and later chips, there are separated mac/mdio resets, but
resetting the entire gmac block with register values requires both
mac_reset and mdio_reset to be asserted together.

Add support for optional mdio reset so that we can do a full reset
if needed.

This patch also replaced deprecated devm_reset_control_get for
mac reset.

To use this feature, the following is needed:
 1. drop "simple-mfd" compatible to register mdio0 after gmac init
    so that mdio registers aren't reset after initialization.
 2. move mdio reset from mdio-bus to its parent eth node.

NOTE: This can't be applied on gmac1 with builtin switch since we
haven't add a feature to defer probe if phy connection failed.

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