mmc: use aligned buf in block_mmc_fill_write

On some platform(like rk3399 etc) would prefer aligned buf. So use
aligned buf in block_mmc_fill_write.

BUG=b:63407238
TEST=build and boot

Change-Id: I8119dc568250b553ecca367c35b1003f6ba44002
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-on: https://chromium-review.googlesource.com/652087Reviewed-by: Julius Werner <jwerner@chromium.org>

src/drivers/storage/mmc.c

@@ -1306,7 +1306,7 @@ lba_t block_mmc_fill_write(BlockDevOps *me, lba_t start, lba_t count,
 
 	uint64_t buffer_bytes = buffer_lba * block_size;
 	uint64_t buffer_words = buffer_bytes / sizeof(uint32_t);
-	uint32_t *buffer = xmalloc(buffer_bytes);
+	uint32_t *buffer = xmemalign(ARCH_DMA_MINALIGN, buffer_bytes);
 	uint32_t *ptr = buffer;
 
 	for ( ; buffer_words ; buffer_words--)