Revert "dt-bindings: remoteproc: pru: Update bindings for supporting rpmsg"

This reverts commit 04c472a2.

The plain text-format document update for the rpmsg support in the PRU
remoteproc binding has been replaced by an equivalent upstream preferred
YAML-format binding document in commit 1563d95e ("dt-bindings:
remoteproc: pru: Update bindings for supporting rpmsg"), so revert the
above text-based binding update patch.

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