mmc: core: Add capability to avoid 3.3V signaling

Some SD host controllers cannot handle extended use of 3.3V signaling.
To accommodate these controllers, add a capability that requires us to
negotiate the voltage down from 3.3V during card initialization.

Signed-off-by: Kyle Roeschley <kyle.roeschley@ni.com>
Signed-off-by: Jennifer Dahm <jennifer.dahm@ni.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>