dt-bindings: remoteproc: Add binding doc for PRU cores in the PRU-ICSS

The Programmable Real-Time Unit and Industrial Communication Subsystem
(PRU-ICSS or simply PRUSS) on various TI SoCs consists of dual 32-bit
RISC cores (Programmable Real-Time Units, or PRUs) for program execution.

The K3 AM65x amd J721E SoCs have the next generation of the PRU-ICSS IP,
commonly called ICSSG. The ICSSG IP on AM65x SoCs has two PRU cores,
two auxiliary custom PRU cores called Real Time Units (RTUs). The K3
AM65x SR2.0 and J721E SoCs have a revised version of the ICSSG IP, and
include two additional custom auxiliary PRU cores called Transmit PRUs
(Tx_PRUs).

This patch adds the bindings for these PRU cores. The binding covers the
OMAP architecture SoCs - AM33xx, AM437x and AM57xx; Keystone 2 architecture
based 66AK2G SoC; and the K3 architecture based SoCs - AM65x and J721E. The
Davinci based OMAPL138 SoCs will be covered in a future patch.

Reviewed-by: Rob Herring <robh@kernel.org>
Co-developed-by: Roger Quadros <rogerq@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>
Link: https://lore.kernel.org/r/20201208141002.17777-2-grzegorz.jaszczyk@linaro.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>