raspberry pi: add model b definition

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>
arm: use raspberry pi platform

As it is the only currently supported ARM platform. It defaults to RPi

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>
raspberry pi: add doc for model b

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>