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.