remoteproc: replace BSD blurb with SPDX identifier in remoteproc.h

remoteproc.h contains the full BSD-3-Clause license blurb but is missing
an SPDX-License-Identifier tag.

Replace the BSD-3-Clause license blurb with the SPDX-License-Identifier
tag, the standard kernel representation, avoiding duplicate license text.

Signed-off-by: Mukesh Ojha <mukesh.ojha@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260803114331.3277263-4-mukesh.ojha@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>