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>