ARM: shmobile: INTC External IRQ pin driver on sh73a0

Adjust the sh73a0 IRQ code to make use of the
INTC External IRQ pin driver for external
interrupt pins IRQ0 -> IRQ31.

This removes quite a bit of special-case code
in intc-sh73a0.c but the number of lines get
replaced with platform device information in
setup-sh73a0.c. The PFC code is also adjusted
to make gpio_to_irq() return the correct
interrupt number.

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