ASoC: rockchip: i2s: add 8 channels capture support

support max 8 channels capture, please add property
'rockchip,capture-channels' in dts to enable this,
if not, support 2 channels capture default.

Signed-off-by: Sugar Zhang <sugar.zhang@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>