ARM: dts: sunxi: Add fixed 5V regulator

Most if not all boards we've seen have a fixed 5V regulator, which is
the main power supply and/or fixed output of the PMIC.

Add this one to the common regulators DTSI.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>