[POWERPC] Bamboo zImage wrapper

Add a bootwrapper for the AMCC 440EP Bamboo Eval board.  This also adds a
common fixup_clock function for all 440EP(x) chips.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>