ARM: mcpm: use -st dsb option prior to sev instructions

In a similar manner to our spinlock implementation, mcpm uses sev to
wake up cores waiting on a lock when the lock is unlocked. In order to
ensure that the final write unlocking the lock is visible, a dsb
instruction is executed immediately prior to the sev.

This patch changes these dsbs to use the -st option, since we only
require that the store unlocking the lock is made visible.

Acked-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Dave Martin <dave.martin@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>