mmc: dw_mmc: zx: Initial support for ZX mmc controller

This platform driver adds initial support for the DW host controller
found on ZTE SoCs.

It has been tested on ZX296718 EVB board currently. More support on
timing tuning will be added when hardware is available.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>