scsi: ufs: add hot plug support for removable UFS cards

Now that we can have the removable UFS cards which can be plugged in/out
at runtime, we need to add the support for gpio based card
insertion/removal event detection support. But this is bit complicated
as current platforms have the combo card slot which can either take UFS
or SD card at one time and there is a single GPIO assigned for card detect
event. This means we need some module which takes the responsibility of
controlling (configuration, interrupt handling) this shared card detection
GPIO and pass card detection event to both UFS and SD card drivers so they
can probe to see what card is inserted. Kernel already have the extcon-gpio
module which can be used as gpio control module and can pass the card
detection event to both UFS & SD card drivers. So UFS driver registers the
notification callback with extcon driver and tries to detect/remove the UFS
card.

Change-Id: Id65156289c2a722ac4a501020fdbfe6f97a354a9
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
[cang@codeaurora.org: resolved trivial merge conflicts]
Signed-off-by: Can Guo <cang@codeaurora.org>