ARM: dts: msm: enable hot plug support of SD cards on sdm845

We need to add the support for gpio based card insertion/removal event
detection support. But this is bit complicated as SDM845 boards have
the combo card slot which can either take SD or UFS 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 MMC and UFS card drivers so they can probe to see what card
is inserted. Kernel already have the extcon-gpio module which can be used
a gpio control module and can pass the card detection event to both MMC &
UFS card drivers. So we are adding the extcon-gpio node which gets used
by SD card and later on by UFS card as well.

Change-Id: I93a6d0fd17b9c4cbe3e736cae7dbb5e9e3aa88ce
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>