spi: s3c64xx: add SPDX identifier

Replace the original license statement with the SPDX identifier.

Signed-off-by: Andi Shyti <andi.shyti@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Philippe Ombredanne <pombredanne@nexb.com>
Signed-off-by: Mark Brown <broonie@kernel.org>