mmc: dw_mmc: Add support for MIPS

There are upcoming MIPS SoCs with dw_mmc hosts.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>