mmc: dw_mmc: Add support for SOCFPGA's platform specific implementation

Like the rockchip, Altera's SOCFPGA platform specific implementation of the
dw_mmc driver requires using the HOLD register for SD commands. This patch
renames dw_mci_rockchip_prepare_command to dw_mci_pltfm_prepare_command so
that SOCFPGA and Rockchip can use it.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Acked-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Tested-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <chris@printf.net>