msm: peripheral-loader: Fix double irq enable

Check device tree for the proxy_unvote gpio only if the pil_desc
defines the proxy_unvote op.

This fixes an issue where the proxy_unvote irq was erroneously
enabled twice by two pil_desc, resulting in a kernel warning.

Change-Id: I07b4377c59a2908caea00b184cc0ef6adfd8f1fb
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>