ARM: shmobile: r8a7778: add r8a7778_init_irq_extpin()

This patch adds r8a7778_init_irq_extpin() for IRQ0 - IRQ3.
But this patch doesn't enable DT settings on r8a7778.dts,
because R8A7778 chip external IRQ depends on
IRQ0 - IRQ3 pin encoding which came from platform board
implementation.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>