ASoC: samsung: i2s: Convert to single component with multiple DAIs

This patch includes minimal changes as a prerequisite for adding support
for the Exynos secondary I2S interface as second DAI of the I2S component.
Doing it that way allows to avoid problems as indicated in commmit
6b01e0365b1689 ("ASoC: samsung: i2s: disable secondary DAI until it gets fixed")

The samsung_i2s_get_pri_dai() helper added in this patch is temporary and
will be removed in one of subsequent patches.
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>