ASoC: samsung: i2s: Convert to SPDX License Indentifier

Replace GPL v2.0 license statements with SPDX license identifier.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>