ARM: shmobile: r8a7791 IRQC platform device support

Add a platform device for the r8a7791 IRQC hardware
driving IRQ pins IRQ0 to IRQ9. The Linux interrupt
number is statically assigned to allow board code
written in C to make use of static interrupt numbers.

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