dt-bindings: remoteproc: Add Keystone DSP remoteproc binding

Add the device tree bindings document for the Texas Instrument's
Keystone 2 DSP remoteproc devices.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Sam Nelson <sam.nelson@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>