net: stmmac: dwmac-rk: Use standard devicetree property for phy regulator

Currently, dwmac-rk uses a custom propety "phy_regulator" to get the name of the
right regulator to use to power on or power off the phy. This commit converts the
driver to use phy-supply devicetree property and the corresponding API, it cleans
the code a bit and make it simpler to maintain. This also replaces the property
phy_regulator by the standard property phy-supply in rk3288-evb-rk808.dts.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>