drivers/mmc: make mmc_ext_csd aligned with 16 char

DMA is always used in mmc driver. So the buffer address should
always follow the DMA limitation.

There're same requirement in mmc_read_blocks()/mmc_write_blocks()
on parameter buf. Since parameter buf comes from io_block driver,
it's already handled in io_block driver.

At here, just make the minimum address alignment on 16 chars.
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>

drivers/mmc/mmc.c

@@ -24,7 +24,7 @@
 static const struct mmc_ops *ops;
 static unsigned int mmc_ocr_value;
 static struct mmc_csd_emmc mmc_csd;
-static unsigned char mmc_ext_csd[512] __aligned(4);
+static unsigned char mmc_ext_csd[512] __aligned(16);
 static unsigned int mmc_flags;
 static struct mmc_device_info *mmc_dev_info;
 static unsigned int rca;