ASoC: samsung: i2s: Document clocks macros

Document clocks macros with their description
from 'Documentation/devicetree/bindings/sound/samsung-i2s.txt'

Signed-off-by: Maciej Falkowski <m.falkowski@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>