mmc: core: Add DT-bindings for MMC_CAP2_FULL_PWR_CYCLE

The DT-binding for MMC_CAP2_FULL_PWR_CYCLE, is used to indicate whether
it is possible to perform a full power cycle of the card.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>