mmc: tmio: Add UHS-I mode support

Based on work by Shinobu Uehara and Ben Dooks. This adds the voltage
switch operation needed for all UHS-I modes, but not the tuning needed
for SDR-104 which will come later.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>