feat(allwinner): allow to skip PMIC regulator setup

feat(allwinner): allow to skip PMIC regulator setup

For somewhat historical reasons we are doing some initial PMIC regulator
setup in BL31, as U-Boot does not (yet) have a PMIC driver. This worked
fine so far, but there is at least one board (OrangePi 3) that gets upset,
because the Ethernet PHY needs some *coordinated* bringup of *two*
regulators.

To avoid custom hacks, let's introduce a build option to keep doing the
regulator setup in TF-A. Defining SUNXI_SETUP_REGULATORS to 0 will break
support for some devices on some boards in U-Boot (Ethernet and HDMI),
but will allow to bring up the OrangePi 3 in Linux correctly. We keep
the default at 1 to not change the behaviour for all other boards.

After U-Boot gained proper PMIC support at some point in the future, we
will probably change the default to 0, to get rid of the less optimal
PMIC code in TF-A.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: Ie8e2583d0396f6eeaae8ffe6b6190f27db63e2a7