dw_mmc: turn on the IO supply

Fixes the microSD slot on the ASUS C201.

Signed-off-by: Urja Rannikko <urjaman@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com<mailto:peng.fan@nxp.com>>