dt-bindings: soc: ti: Add TI PRUSS bindings

This patch adds the bindings for the Programmable Real-Time Unit
and Industrial Communication Subsystem (PRU-ICSS) present on various
TI SoCs. The IP is present on multiple TI SoC architecture families
including the OMAP architecture SoCs such as AM33xx, AM437x and
AM57xx; and on a Keystone 2 architecture based 66AK2G SoC. It is
also present on the Davinci based OMAPL138 SoCs and K3 architecture
based AM65x and J721E SoCs as well.

The IP has a number of sub-modules some of which are represented as
their own devices. This binding covers only the top-level sub-system
devices, and some sub-modules like MDIO, MII_RT (Ethernet MII_RT module
with MII ports) and IEP (Industrial Ethernet Peripheral). The remaining
sub-modules bindings shall be defined in the respective driver
subsystem bindings folders. Couple of full examples have also been
added demonstrating the devices on AM335x and AM437x SoCs.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Grzegorz Jaszczyk <grzegorz.jaszczyk@linaro.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>