Documentation: dt: Update PRUSS remoteproc bindings

The PRUSS remoteproc binding document has been updated to include
additional details about various sub-modules used for Industrial
Communication purposes, covering the MDIO, MII_RT and the IEP
sub-modules for now.

The binding status is also marked as unstable as the binding has
been updated only to include the details about the Ethernet usecases.

Signed-off-by: Suman Anna <s-anna@ti.com>