ASoC: soc-pcm: DPCM cares BE format

Current DPCM is caring only FE format. but it will be no sound
if FE/BE was below style, and user selects S24_LE format.

        FE: S16_LE/S24_LE
        BE: S16_LE

DPCM can rewrite the format, so basically we don't want to
constrain with the BE constraints. But sometimes it will be trouble.
This patch adds new .dpcm_merged_format on struct snd_soc_dai_link.
DPCM will use FE / BE merged format if .struct snd_soc_dai_link
has it. We can have other .dpcm_merged_xxx in the future

    .dpcm_merged_foramt
    .dpcm_merged_rate
    .dpcm_merged_chan

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Keita Kobayashi <keita.kobayashi.ym@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>