MAINTAINERS: add entry for LICENSES and SPDX stuff

Thomas and I seem to have become the "unofficial" maintainers for these
files and questions about SPDX things.  So let's make it official.

Reported-by: "Darrick J. Wong" <darrick.wong@oracle.com>
Grumpily-acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Darrick J. Wong <darrick.wong@oracle.com>
Link: https://lore.kernel.org/r/20190827195310.GA30618@kroah.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>