ARM: shmobile: r8a7778: remove r8a7778_init_irq()

This patch removes r8a7778_init_irq(), since no-one is using it.
And now, there is no reason to have r8a7778_init_irq_common().
r8a7778_init_irq_dt() includes it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>