docs(allwinner): add SUNXI_SETUP_REGULATORS build option

Document the newly introduced SUNXI_SETUP_REGULATORS build option, that
allows to disable PMIC regulator setup at build time.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: Ie5fb0b7220426b67cfffc95df4cabb31a6ec174a