mmc: core: Add the vmmc/vmmcq regulator info

The core MMC code adds two (optional) regulator properites that drivers
should use to get their supplies. This is not documented anywhere so add
information on it.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>