[POWERPC] 4xx: Add mfspr/mtspr inline macros to 4xx bootwrapper

The 4xx bootwrapper occasionally needs to access SPR registers,
this adds mfspr/mtspr wrappers to it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>