ARM: shmobile: r8a7778: Make r8a7778_init_irq_dt() static

As of commit 4baadb9e ("ARM: shmobile: r8a7778: remove obsolete
setup code"), r8a7778_init_irq_dt() is no longer used outside
setup-r8a7778.c.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>