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

The Programmable Real-Time Unit Subsystem (PRUSS) consists of dual
32-bit RISC cores (Programmable Real-Time Units, or PRUs) for program
execution. This patch adds the bindings for these PRU cores.

The binding currently covers the OMAP architecture SoCs - AM33xx,
AM437x and AM57xx; and a Keystone 2 architecture based 66AK2G SoC.
The Davinci based OMAPL138 SoCs and K3 architecture based AM65x SoCs
will be covered in a subsequent patch.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>