powerpc/405ex: provide necessary fixup function to support cuImage

powerpc/405ex: provide necessary fixup function to support cuImage

For cuImage format it's necessary to provide clock fixups since u-boot will
not pass necessary clock frequency into the dtb included into cuImage so we
implement the clock fixups as defined in the technical documentation for the
board and update header file with the basic register definitions.

Signed-off-by: Tiejun Chen <tiejun.chen@windriver.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>