ARM: mach-shmobile: r8a7779: Allow initialisation of GIC by DT

This allows the GIC interrupt controller of the  r8a7779 SoC to be
initialised using a flattened device tree blob.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

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v3
* Fix copy-paste error and use unique reg values for each CPU

v2
As suggested by Mark Rutland
* Add reg and device_type to cpus
* Remove #address-cells from gic