MX28: MMC: Avoid DMA DCache race condition

This patch prevents dcache-related problem. The problem manifested
itself on the SPI driver, this is just a port to the MMC driver.

The scenario is the same. In case an "mmc read" is issued to a
buffer which was written right before it and data cache is enabled,
the cache eviction might happen during the DMA transfer into the
buffer, therefore corrupting the buffer. Clear any cache lines that
might contain the buffer to prevent such issue.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Stefano Babic <sbabic@denx.de>