mmc: dw_mmc-pltfm: add Rockchip variant

Cortex-A9 SoCs from Rockchip use a slightly modified variant of dw_mmc
controllers that seems to require the SDMMC_CMD_USE_HOLD_REG bit to
always be set.

There also seem to be no other modifications (additional register etc)
present, so to keep the footprint low, add this small variant to the
pltfm driver.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>