soc: qcom: pil: Fix the parsing of the proxy-unvote irq

"proxy_unvote" irq is a name under property "interrupt-names" so
fix the device tree reading using the correct API.

Change-Id: I06cb9e33ac052def99adeb40c381ce3a073bbb18
Signed-off-by: Runmin Wang <runminw@codeaurora.org>