mmc: sdhci: convert align buffer to a coherent allocation

Rather than using the streaming API, use the coherent allocator to
provide this memory, thereby eliminating cache flushing of it each
time we map and unmap it.

CRs-fixed: 684185
Change-Id: I615133da06a04fa23c6bca4078e484299147eb34
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>