ARM: u300: set up board power from device tree

This adds support for setting up the board power from the
device tree on the U300. We use a board-specific node in the
device tree for the S365 board and bind a regulator for the
board power to this node.

Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>