ARM: dts: msm: Add interrupt support to virtio spmi

The spmi pmic arbiter, as the root of all pmic clients,
becomes a virtio mmio device but not a platform device.
It's also an interrupt controller.

Change-Id: Ica19411cb5b0c7178faa093dac3627d1dc42221c
Signed-off-by: Yimin Peng <yiminp@codeaurora.org>