ARM: shmobile: r8a7740: Instantiate GIC from C board code in legacy builds

As of commit 9a1091ef

 ("irqchip: gic: Support hierarchy irq
domain."), armadillo-legacy hangs during boot with:

     sh-tmu.0: ch0: used for clock events
     sh-tmu.0: ch0: used for periodic clock events
     sh-tmu.0: ch0: failed to request irq 230
     sh-tmu.0: ch1: used as clock source
     sh-cmt-48.1: ch0: failed to request irq 90
     sh-cmt-48.1: ch0: registration failed
    earlytimer: unable to probe sh-cmt-48 early.
    Calibrating delay loop...

This 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 on sh73a0 for kzm9g-legacy.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>