ARM: 7618/1: mmc: mmci: Support MMC_PM_KEEP_POWER

Add MMC_PM_KEEP_POWER to pm_caps so SDIO clients are able
to use this option to prevent power off in suspend.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>