Revert "dt-bindings: remoteproc: Add binding doc for PRU Cores in the PRU-ICSS"

This reverts commit 299339cc.

The plain text-format binding document for the PRU remoteproc device has
been replaced by an equivalent upstream preferred YAML-format binding
document in commit d74629e2 ("dt-bindings: remoteproc: Add binding
doc for PRU cores in the PRU-ICSS"), so revert the original text-based
binding patch.

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