ARM: shmobile: add GPIO IRQ macro

R-Car series gpio_rcar driver can control GPIO IRQ today.
It needs base IRQ number for gpio_rcar_config :: .irq_base
This patch adds macro for GPIO IRQ.
This patch was tested on Bock-W board

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>