dt-bindings: mailbox: Add support for secure proxy threads

Secure Proxy is another communication scheme in Texas Instrument's
devices intended to provide an unique communication path from various
processors in the System on Chip(SoC) to a central System Controller.

Secure proxy is, in effect, an evolution of current generation Message
Manager hardware block found in K2G devices. However the following
changes have taken place:

Secure Proxy instance exposes "threads" or "proxies" which is
primary representation of "a" communication channel. Each thread is
preconfigured by System controller configuration based on SoC usage
requirements. Secure proxy by itself represents a single "queue" of
communication but allows the proxies to be independently operated.

Each Secure proxy thread can uniquely have their own error and threshold
interrupts allowing for more fine control of IRQ handling.

Provide an hardware description of the same for device tree
representation.

See AM65x Technical Reference Manual (SPRUID7, April 2018)
for further details: http://www.ti.com/lit/pdf/spruid7



Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>