ARM: shmobile: r8a7779: split r8a7779_init_irq_extpin() for DT

r8a7779 INTC needs IRL pin mode settings to determine
behavior of IRQ0 - IRQ3, and r8a7779_init_irq_extpin()
is controlling it via irlm parameter.
But this function registers renesas_intc_irqpin driver
if irlm was set, and this value depends on platform.
This is not good for DT.
This patch splits r8a7779_init_irq_extpin() function
into "mode settings" and "funtion register" parts

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