ARM: u300: device tree support for the timer

This adds device tree support for the U300 timer, by making
the memory base offset and IRQ dynamically assigned, then
optionally looking them up from the device tree.

Since the timer needs to be registered before any platform
devices are created, we will go into the device tree and look
up the "/timer@c0014000" node and read our base address and
IRQ from there.

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