staging: mt7621-pinctrl: add SPDX identifier

It's good to have SPDX identifiers in driver files to make it easier to
audit the kernel tree for correct licenses.

Fix up the one of staging mt7621-pinctrl file to have a proper SPDX
identifier, based on the license text in the file itself. The SPDX
identifier is a legally binding shorthand, which can be used instead of
the full boiler plate text.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>