[POWERPC] 4xx: Add dcri_clrset() for locked read/modify/write functionality

This adds dcri_clrset() macro which does read/modify/write
on indirect dcr registers while holding indirect dcr lock.

Signed-off-by: Valentine Barshak <vbarshak@ru.mvista.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>