Switch to use the more robust common mmc_regulator_set_vqmmc()
function in MMC core which set the target voltage as close as
possible to target voltage.

We did not re-factor the whole sdhci_start_signal_voltage_switch()
cause we want to keep the original signal switch order between host
and card to avoid potential break.
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>