sunxi: remove CONFIG_MACPWR

sunxi: remove CONFIG_MACPWR

The CONFIG_MACPWR Kconfig symbol is used to point to a GPIO that enables
the power for the Ethernet "MAC" (mostly PHY, really).
In the DT this is described with the phy-supply property in the MAC DT
node, pointing to a (GPIO controlled) regulator. Since we need Ethernet
only in U-Boot proper, and use a DM driver there, we should use the DT
instead of hardcoding this.

Add code to the sun8i_emac and sunxi_emac drivers to check the DT for
that regulator and enable it, at probe time. Then drop the current code
from board.c, which was doing that job before.
This allows us to remove the MACPWR Kconfig definition and the respective
values from the defconfigs.

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