pata_bk3710: clarify license version and use SPDX header

- clarify license version (it should be GPL 2.0)
- use SPDX header

Acked-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Tejun Heo <tj@kernel.org>