ARM: add basic support for the Broadcom BCM2835 SoC

ARM: add basic support for the Broadcom BCM2835 SoC

This SoC is used in the Raspberry Pi, for example.

For more details, see:
http://www.broadcom.com/products/BCM2835
http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf.

Initial support is enough to boot to a serial console, execute a minimal
set of U-Boot commands, download data over a serial port, and boot a
Linux kernel. No storage or network drivers are implemented.

GPIO driver originally by Vikram Narayanan <vikram186@gmail.com>
with many fixes from myself.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>