irqchip: intc-irqpin: Add support for shared interrupt lines

On some hardware we don't have a 1-1 mapping from the external
interrupts coming from INTC to the GIC SPI pins. We can however
share lines to demux incoming IRQs on these SoCs.

This patch enables the intc_irqpin driver to detect requests for shared
interrupt lines and demuxes them properly by querying the INTC INTREQx0A
registers.

If you need multiple shared intc_irqpin device instances, be sure to mask
out all interrupts on the INTC that share the one line before you start
to register them. Else you run into IRQ floods that would be caused by
interrupts for which no handler has been set up yet when the first
intc_irqpin device is registered.

Signed-off-by: Bastian Hecht <hechtb+renesas@gmail.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>