ASoC: rockchip: i2s: Make playback/capture optional

There are some controllers which support playback only or
capture only. so, make it optional. and initial capability
by 'dma-names' of DT.

Signed-off-by: Sugar Zhang <sugar.zhang@rock-chips.com>
Link: https://lore.kernel.org/r/1629950562-14281-4-git-send-email-sugar.zhang@rock-chips.com


Signed-off-by: Mark Brown <broonie@kernel.org>