mmc: dw_mmc: add switch_voltage

switch_voltage is required on some platform since special register accessing

Signed-off-by: Jorge A. Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Dan Yuan <yuandan@hisilicon.com>
Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>