ARM: shmobile: r8a7779: Instantiate GIC from C board code in legacy builds

As of commit 9a1091ef ("irqchip: gic: Support hierarchy irq
domain."), Marzen legacy hangs during boot with:

   Image Name:   'Linux-3.19.0-rc4'
   Image Type:   ARM Linux Kernel Image (uncompressed)
   Data Size:    3445880 Bytes = 3.3 MiB
   Load Address: 60008000
   Entry Point:  60008000
   Verifying Checksum ... OK
   Loading Kernel Image ... OK
OK

Starting kernel ...

Enabling DEBUG_LL does not seem to change the situation, however this
patch by itself fixes this issue and re-enables normal boot.

This issue happens because the IRQ numbers of the GIC are now virtual,
and no longer match the hardcoded hardware IRQ numbers in the platform
board code.

To fix this, instantiate the GIC from platform board code when compiling
a legacy kernel, like is done for the sh73a0, r8a7740 and r8a7778 legacy code.

Follows same style as the r8a7740 legacy GIC fix by Geert Uytterhoeven,
thanks to him for the initial work.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>