[POWERPC] Add support for 750CL Holly board

Add PowerPC 750 Holly/Hickory platform support

Signed-off-by: Stephen Winiecki <stevewin@us.ibm.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>