ARM: add Raspberry Pi model B board, using BCM2835 SoC

ARM: add Raspberry Pi model B board, using BCM2835 SoC

The Raspberry Pi model B uses the BCM2835 SoC, has 256MB of RAM,
contains an SMSC 9512 USB LAN/Hub chip, and various IO connectors.
For more details, see http://www.raspberrypi.org/.

Various portions (cache enable, MACH_TYPE setup, RAM size limit, stack
relocation to top of RAM) extracted from work by:
Oleksandr Tymoshenko <[email protected]>.

GPIO driver enablement by Vikram Narayanan <[email protected]>.

Signed-off-by: Stephen Warren <[email protected]>
Acked-by: Tom Rini <[email protected]>