ASoC: zx-i2s: introduce pclk for zx2967 family

The pclk is necessary for zx2967 I2S controller. the driver
currently doesn't handle it. This is something we need to fix.

In turn, the driver supports zx296718's I2S controller.

By the way, this patch also change the clock name from tx to wclk
to make it clear.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Reviewed-by: Shawn Guo <shawnguo@kernel.org>
Reviewed-by: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>