[POWERPC] 4xx: Add platform support for the AMCC Yosemite board

The AMCC 440EP Yosemite board is very similar to the original AMCC Bamboo
board.  This adds a YOSEMITE option to Kconfig, and reuses the existing
bamboo board support in the kernel.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>