ARM: dts: msm: add ehci interrupt to MPM interrupt mapping for MSM8994

When enterring suspend, msm_ehci_host (irq 172) prevents the
system from voting for XO shutdown as it is not being programmed
into the MPM. Add a mapping from this interrupt to MPM pin 55.

Change-Id: Idd2263df5995079853818127330acf2d626b8abd
Signed-off-by: Jack Pham <jackp@codeaurora.org>