mmc: sdhci: Set SDHCI_POWER_ON with external vmmc

Host controllers lacking the required internal vmmc regulator may still
follow the spec with regard to the LSB of SDHCI_POWER_CONTROL.  Set the
SDHCI_POWER_ON bit when vmmc is enabled to encourage the controller to
to drive CMD, DAT, SDCLK.

This fixes a regression observed on some Qualcomm and Nvidia boards
caused by 52221610 mmc: sdhci: Improve external VDD regulator support.

Fixes: 52221610

 (mmc: sdhci: Improve external VDD regulator support)
Signed-off-by: Tim Kryger <tim.kryger@gmail.com>
Tested-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>