irqchip: gic-v3: Add panic handler to save registers

Save gic-v3 distributor registers on panic. Allows for inspection of
gic-v3 state at the time of a panic.

Change-Id: I3236577161abab4e292a01254e1e1ecb50bb38de
Signed-off-by: Cassidy Burden <cburden@codeaurora.org>