ARM: shmobile: sh73a0: Remove sh73a0_init_irq_dt()

This is not needed as irq_set_wake is
only used for suspend to ram which is not
a requirement for bringing up boards using DT.

Reported-by: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>