MIPS: ralink: Switch pinmux.h to SPDX header

The original license text had a typo ("publishhed") which would be
likely to confuse automated licensing auditing tools. Let's just switch
to SPDX instead of fixing the wording.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org