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 <mail@david-bauer.net>
Cc: John Crispin <john@phrozen.org>
Signed-off-by: Sander Vanheule <sander@svanheule.net>
Acked-by: David Bauer <mail@david-bauer.net>