sunxi: remove CONFIG_SATAPWR

sunxi: remove CONFIG_SATAPWR

The CONFIG_SATAPWR Kconfig symbol was used to point to a GPIO that
enables the power for a SATA harddisk.
In the DT this is described with the target-supply property in the AHCI
DT node, pointing to a (GPIO controlled) regulator. Since we need SATA
only in U-Boot proper, and use a DM driver for AHCI there, we should use
the DT instead of hardcoding this.

Add code to the sunxi AHCI driver 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 SATAPWR Kconfig definition and the
respective values from the defconfigs.
We also select the generic fixed regulator driver, which handles those
GPIO controlled regulators.

Please note that the OrangePi Plus is a bit special here, it's a H3
board without native SATA, but with a USB-to-SATA bridge. The DT models
the SATA power via a VBUS supply regulator, which we don't parse yet in
the USB PHY driver. Use the hardcoded CONFIG_USB3_VBUS_PIN for that
board meanwhile.

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