ARM: realview: basic device tree implementation

This implements basic device tree boot support for the RealView
platforms, with a basic device tree for ARM PB1176 as an example.

The implementation is done with a new DT-specific board file
using only pre-existing bindings for the basic IRQ, timer and
serial port drivers. A new compatible type is added to the GIC
for the ARM1176.

This implementation uses the MFD syscon handle from day one to
access the system controller registers, and register the devices
using the SoC bus.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rob Herring <robh@kernel.org>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>