there maybe more than one i2s module inside chip, and these i2s modules
have different channels features.

for example: there are 3 i2s in rk3066, one support 8 channels playback
and 2 channels capture, but the others only support 2 channels playback
and 2 channels capture.

in order to compatible with these various chips, we add playback and
capture property to specify these values.

there are default channels configuration in driver: 8 channels playback
and 2 channels capture. if not add property, we use the default values.
Signed-off-by: NSugar Zhang <sugar.zhang@rock-chips.com>
Signed-off-by: NMark Brown <broonie@kernel.org>