thermal: rcar_thermal: convert to SPDX identifiers

As original license mentioned, it is GPL-2.0 in SPDX.
Then, MODULE_LICENSE() should be "GPL v2" instead of "GPL".
See ${LINUX}/include/linux/module.h

	"GPL"           [GNU Public License v2 or later]
	"GPL v2"        [GNU Public License v2]

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>