[POWERPC] Rename 44x bootwrapper

Rename the 44x.c wrapper file to 4xx.c.  This will allow us to add common
functions in a single file that can be shared across all of 4xx.

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