ARM: mach-shmobile: sh73a0: Allow initialisation of GIC by DT

This allows the GIC interrupt controller of the sh73a0 SoC to be
initialised using a flattened device tree blob.

It does not allow the INTC interrupt controller which is also present on
the sh73a0 SoC to be enabled via device tree.  Nor does it handle sharing
of interrupts between the GIC and INTC interrupt controllers.

This limits the usefulness of this code to applications which only wish to
access devices which use interrupts that can be handled by the GIC
interrupt controller. Other applications should, for now, continue using
non-device tree initialisation of the sh72a0 interrupt controllers.

Includes update to use irqchip_init() by Thierry Reding

Cc: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>