ARM: u300: add syscon node

This adds a device tree node for the U300 system controller
and remaps this dynamically instead of using hard-coded
virtual addresses. The board power set-up code is altered
to fetch a reference to the syscon using ampersand <&syscon>
notation. This way of passing a pointer to the syscon will
also be used by the clocks.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>