[ARM] 4711/1: pxa: mmc: move DMA specific code to platform layer

This patch is to move pxamci DMA specific code to corresponding
platform layer because using DRCMRRXMMC/DRCMRTXMMC in pxamci.c makes
the driver code dedicated to platform which is not extensible.

It is applicable to all pxa platforms.

Signed-off-by: Bridge Wu <bridge.wu@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>