phy: add support for phy-supply

Some phys require a phy-supply property that is a phandle to a regulator
that needs to be enabled for phy operations.
Implement basic supply lookup, enable and disabling, if DM_REGULATOR is
available.

Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
[jonas@kwiboo.se:
use regulator_set_enable_if_allowed and disable if power_on ops fails]
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>