ARM: shmobile: kzm9g: Remove init_irq declaration in machine description

Remove redundant irqchip_init() callback. The default case
of NULL will result in invoking irqchip_init() anyway.

Signed-off-by: Magnus Damm <damm@opensource.se>
[ horms+renesas@verge.net.au: Trimmed patch to remove portion
  that updates the sh73a0 SoC and altered the subject to
  use the same format as the patch that updates the sh73a0. ]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>