ARM: keystone: Drop the un-necessary dsb from keystone_cpu_smc()

This was added because of some legacy reasons from OMAP SOCs but
after testing and verifying with the keystone hardware folks, the
dsb in keystone_cpu_smc() is not necessary.

So drop it.

Reported-by: Dave Martin <dave.martin@linaro.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>