ARM: shmobile: r8a7779: Configure IRLM mode via DT

Adjust the r8a7779 SoC DTS and the Marzen Reference
C board code to use DTS only for INTC-IRQPIN IRLM setup.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>