irqchip/imgpdc: Use resource_size function on resource object

drivers/irqchip/irq-imgpdc.c:327:20-23: WARNING: Suspicious code.
		resource_size is maybe missing with res_regs

Generated by: scripts/coccinelle/api/resource_size.cocci

Signed-off-by: Vasyl Gomonovych <gomonovych@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: marc.zyngier@arm.com
Cc: jason@lakedaemon.net
Link: https://lkml.kernel.org/r/1511215361-8279-1-git-send-email-gomonovych@gmail.com