MIPS: ralink: mt7628a.dtsi: Add SPDX GPL-2.0 license identifier

As done in commit b2441318

 ("License cleanup: add SPDX GPL-2.0
license identifier to files with no license"), this patch adds the SPDX
license identifier to mt7628a.dtsi, which is currently still missing
this identifier.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Harvey Hunt <harveyhuntnexus@gmail.com>
Cc: John Crispin <john@phrozen.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org