irqchip: intc-irqpin: Initial DT support

Add initial DT support to the INTC External IRQ Pin
driver. At this point only hardware with 4-bit wide
sense registers is supported via DT.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>