ARM: 7921/1: mcpm: remove redundant dsb instructions prior to sev

sync_cache_w already includes a dsb, so we can just use sev() directly
then following a cache-sync.

Acked-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>