ARM: dts: msm: Add the gpio-shutdown-ack entry to modem on msm8996

After sending a sysmon shutdown request to the SSCTL service on
the modem, wait for the service to either set the shutdown_ack
smp2p bit or to send the indication message back.

Change-Id: Id757f75d256bb240f08f9b8ebda548ed60c04f34
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>