mmc: core: Handle both poweroff notification types for eMMC

Depending on the context of the operation while powering down the card,
either POWER_OFF_NOTIFY_SHORT or POWER_OFF_NOTIFY_LONG will be used. In
suspend context a short timeout is preferred while a long timeout would
be acceptable in a shutdown/hibernation context.

We add a new parameter to the mmc_suspend function so we can provide an
indication of what notification type to use.

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