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.