mmc: dw_mmc-socfpga: Remove the SOCFPGA specific platform for dw_mmc

It turns now that the only really platform specific code that is needed for
SOCFPGA is using the SDMMC_CMD_USE_HOLD_REG in the prepare_command function.
Since the Rockchip already has this functionality, re-use the code that is
already in dw_mmc-pltfm.c.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.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>