ARM: shmobile: INTC External IRQ pin driver on r8a7779

Update the r8a7779 IRQ code to make use of the
INTC External IRQ pin driver for external
interrupt pins IRQ0 -> IRQ3.

The r8a7779 SoC can like older SH SoCs configure
to use the IRQ0 -> IRQ3 signals as individual
interrupts or a combined IRL mode.

Without this patch the r8a7779 SoC code does
not fully support external IRQ pins in individual
IRQ mode. The r8a7779 PFC code does not yet have
gpio_to_irq() support so no need to update such
code.

At this point the DT reference implementations
are not covered. In the future such code shall
tie in the INTC External IRQ pin driver via
DT, so this kind of verbose code is not needed
for the long term DT case.

Signed-off-by: Magnus Damm <damm@opensource.se>
Tested-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>