irqchip: gic: Add kconfig option for gic panic handler

Don't handle panic in gic driver unless debugging to save space. Enabling
gic panic handler allows developers to inspect gic from a RAM dump, but
requires 12 KiB of stack space for storing the registers.

Change-Id: Ic6d8ed956a09bc2d6608174244ecab25fb4ae216
Signed-off-by: Cassidy Burden <cburden@codeaurora.org>