ASoC: fsl ssi doc: Move optional properties to the correct section

Properties 'fsl,ssi-asynchronous', 'fsl,playback-dma' and 'fsl,capture-dma'
are optional, so move them under the optional section of the document.

While at it mention that 'fsl,playback-dma' and 'fsl,capture-dma'
only apply to Power Architecture.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Timur Tabi <timur@tabi.org>
Signed-off-by: Mark Brown <broonie@kernel.org>