dt-bindings: remoteproc: pru: Document application node bindings

Add documentation for the common properties that can be used by the
PRU consumer/application nodes and supported by the PRU remoteproc
driver. These are used to configure the PRU hardware for specific
user applications.

The application nodes themselves should define their own bindings.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
[s-anna@ti.com: some binding updates]
Signed-off-by: Suman Anna <s-anna@ti.com>