net: sunxi_emac: chase DT nodes to find PHY regulator

net: sunxi_emac: chase DT nodes to find PHY regulator

At the moment the sun4i EMAC driver relies on hardcoded CONFIG_MACPWR
Kconfig symbols to enable potential PHY regulators. As we want to get rid
of those, we need to find the regulator by chasing up the DT.

The sun4i-emac binding puts the PHY regulator into the MDIO node, which
is the parent of the PHY device. U-Boot does not have (and does not
need) an MDIO driver, so we need to chase down the regulator through the
EMAC node: we follow the "phy-handle" property to find the PHY node,
then go up to its parent, where we find the "phy-supply" link to the
regulator. Let U-Boot find the associated regulator device, and put that
into the private device struct, so we can find and enable the regulator
at probe time, later.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Sam Edwards <CFSworks@gmail.com>
Reviewed-by: Samuel Holland <samuel@sholland.org>