mmc: core: Update PON based on the system state

As per eMMC specification, the PON (Power Off Notification)
must be sent by host to the card before turning off the power.
This will allow card to prepare itself for the power off and
may even reduce the initialization of eMMC upon next boot-up.

Send long PON during system power off and send short PON during
system reboot to reduce the reboot latency.

Change-Id: If4188b8b80aaa0e6c4e00e1807aa9589d5e7efdb
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Krishna Konda <kkonda@codeaurora.org>