ARM: l2x0: use -st dsb option for ordering writel_relaxed with unlock

writel_relaxed and spin_unlock are both store operations, so we only
need to enforce store ordering in the dsb.
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm/mm/cache-l2x0.c

@@ -290,7 +290,7 @@ static void l2x0_disable(void)
 	raw_spin_lock_irqsave(&l2x0_lock, flags);
 	__l2x0_flush_all();
 	writel_relaxed(0, l2x0_base + L2X0_CTRL);
-	dsb();
+	dsb(st);
 	raw_spin_unlock_irqrestore(&l2x0_lock, flags);
 }