sunxi: mctl_mem_matches: Add missing memory barrier

We are running with the caches disabled when mctl_mem_matches gets called,
but the cpu's write buffer is still there and can still get in the way,
add a memory barrier to fix this.

This avoids mctl_mem_matches always returning false in some cases, which
was resulting in:

U-Boot SPL 2015.07 (Apr 14 2016 - 18:47:26)
DRAM: 1024 MiB

U-Boot 2015.07 (Apr 14 2016 - 18:47:26 +0200) Allwinner Technology

CPU:   Allwinner A23 (SUN8I)
DRAM:  512 MiB

Where 512 MiB is the right amount, but the DRAM controller would be
initialized for 1024 MiB.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>

arch/arm/mach-sunxi/dram_helpers.c

@@ -7,6 +7,7 @@
  */
 
 #include <common.h>
+#include <asm/armv7.h>
 #include <asm/io.h>
 #include <asm/arch/dram.h>
 
@@ -31,6 +32,7 @@ bool mctl_mem_matches(u32 offset)
 	/* Try to write different values to RAM at two addresses */
 	writel(0, CONFIG_SYS_SDRAM_BASE);
 	writel(0xaa55aa55, (ulong)CONFIG_SYS_SDRAM_BASE + offset);
+	DSB;
 	/* Check if the same value is actually observed when reading back */
 	return readl(CONFIG_SYS_SDRAM_BASE) ==
 	       readl((ulong)CONFIG_SYS_SDRAM_BASE + offset);