[POWERPC] cell: add shadow registers for pmd_reg

Many of the registers in the performance monitoring unit are write-only.
We need to save a "shadow" copy when we write to those registers so we
can retrieve the values if we need them later.

The new cbe_pmd_shadow_regs structure is added to the cbe_regs_map structure
so we have the appropriate per-node copies of these shadow values.

Signed-off-by: Kevin Corry <kevcorry@us.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>