This patch adds a driver for external IRQ pins connected
to the IRQC hardware block on recent SoCs from Renesas.

The IRQC hardware block is used together with more
recent ARM based SoCs using the GIC. As usual the GIC
requires external IRQ trigger setup somewhere else
which in this particular case happens to be IRQC.

This driver implements the glue code needed to configure
IRQ trigger and also handle mask/unmask and demux of
external IRQ pins hooked up from the IRQC to the GIC.

Tested on r8a73a4 but is designed to work with a wide
range of SoCs. The driver requires one GIC SPI per
external IRQ pin to operate.  Each driver instance
will handle up to 32 external IRQ pins.

The SoCs using this driver are currently mainly used
together with regular platform devices so this driver
allows configuration via platform data to support things
like static interrupt base address. DT support will
be added incrementally in the not so distant future.

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>