LICENSES: Explicitly allow SPDX-FileCopyrightText

LICENSES: Explicitly allow SPDX-FileCopyrightText

Sources already have SPDX-FileCopyrightText (~40 instances) and more
appear on the mailing list, so document that it is allowed.  On the
other hand SPDX defines several other tags like SPDX-FileType, so add
checkpatch rule to narrow desired tags only to two of them - license and
copyright.  That way no new tags would sneak in to the kernel unnoticed.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Joe Perches <joe@perches.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>