mmc: dcache: allocate cache aligned buffer for scr and switch_status

Currently the sd_change_freq function allocates two buffers on the
stack that it passes down to the MMC device driver.  These buffers
could be unaligned to the L1 dcache line size.  This causes problems
when using DMA and with caches enabled.

This patch correctly cache alignes the buffers used for reading the
scr register and switch status values from an MMC device.

Change-Id: Ifa8414f572ef907681bd2d5ff3950285a215357d
Signed-off-by: Anton Staaf <robotboy@chromium.org>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Acked-by: Mike Frysinger <vapier@gentoo.org>