ARM: ux500: Add Device Tree nodes for AB8500 External regulators

The AB8500 has 3 external regulators which are used to control outside
voltage sources. Some of the core AB8500 use these external regulators
as a supply, so they must be obtainable via Device Tree.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>