dt-bindings: irqchip: Add PRU-ICSS interrupt controller bindings

The Programmable Real-Time Unit and Industrial Communication Subsystem
(PRU-ICSS or simply PRUSS) contains an interrupt controller (INTC) that
can handle various system input events and post interrupts back to the
device-level initiators. The INTC can support up to 64 input events on
most SoCs with individual control configuration and h/w prioritization.
These events are mapped onto 10 output interrupt lines through two levels
of many-to-one mapping support. Different interrupt lines are routed to
the individual PRU cores or to the host CPU or to other PRUSS instances.

The K3 AM65x and J721E SoCs have the next generation of the PRU-ICSS IP,
commonly called ICSSG. The ICSSG interrupt controller on K3 SoCs provide
a higher number of host interrupts (20 vs 10) and can handle an increased
number of input events (160 vs 64) from various SoC interrupt sources.

Add the bindings document for these interrupt controllers on all the
applicable SoCs. It covers the OMAP architecture SoCs - AM33xx, AM437x
and AM57xx; the Keystone 2 architecture based 66AK2G SoC; the Davinci
architecture based OMAPL138 SoCs, and the K3 architecture based AM65x
and J721E SoCs.

Co-developed-by: Andrew F. Davis <afd@ti.com>
Co-developed-by: Roger Quadros <rogerq@ti.com>
Co-developed-by: Grzegorz Jaszczyk <grzegorz.jaszczyk@linaro.org>
Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Grzegorz Jaszczyk <grzegorz.jaszczyk@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>