target/arm/whpx: Don't try to sync ARM_CP_CONST registers

ARM_CP_CONST registers ignore writes, so readback assertion fails.

Not asserting now because all entries in the whpx_sreg_match table are
invalid. Needs to be fixed now so that we can fix the table entries.

Signed-off-by: Doug Cook <dcook@microsoft.com>
Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>