arm/dt: 8974: Add MPM interrupt mapping for the SPMI

The SPMI interrupt is connected to MPM and is used for wakeup from system
wide sleep modes. Add interrupt mapping between the MPM pin and the SPMI
interrupt to Krait, so that the MPM driver can configure the MPM pin to
wake up from system wide low power modes.

Change-Id: Ic5499278026ec7108d35e2ea13abbef5e61ec158
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>