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/

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Various portions (cache enable, MACH_TYPE setup, RAM size limit, stack
relocation to top of RAM) extracted from work by:
Oleksandr Tymoshenko <gonzo@bluezbox.com>.

GPIO driver enablement by Vikram Narayanan <vikram186@gmail.com>.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Tom Rini <trini@ti.com>