ath79: ag71xx-mdio: remove phy-mask property

ath79: ag71xx-mdio: remove phy-mask property

The phy-mask property is read by the ag71xx-mdio driver to set the
mii_bus's phy_mask field. On OF platforms, the devicetree is expected to
provide all present ethernet phy-s however, so the phy_mask field is
later set to all-ones. Having a devicetree override is of no use then,
so let's drop it.

Cc: David Bauer <[email protected]>
Cc: John Crispin <[email protected]>
Signed-off-by: Sander Vanheule <[email protected]>
Acked-by: David Bauer <[email protected]>