mmc: dw_mmc: Add support for IMG Pistachio

Add support for the DW MMC host found on the Imagination Pistachio SoC.
Like the DW MMC hosts found on SOCFPGA and Rockchip SoCs, the DW MMC
host on Pistachio requires the use of SDMMC_CMD_USE_HOLD_REG.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>