dt-bindings: remoteproc: Add PRU consumer bindings

Add a YAML binding document for PRU consumers. The binding includes
all the common properties that can be used by different PRU consumer
or 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: Suman Anna <s-anna@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>