irqchip: gic-v3: support to get pending irqs and highprio irq

Add APIs to show pending interrupts in GIC, and next highest
priority interrupt on the current cpu.

Change-Id: Icde27deb7f0a32450ff8f5296e6a175b3559b7af
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>
Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>