MMC: MXS: Convert MXS MMC driver to generic bounce buffer

Implement necessary code to use the generic bounce buffer routines
inside this driver. This replaces the MMC bounce buffer, which is
to be removed.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Andy Fleming <afleming@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>