ASoC: zx-i2s: Add the info of pclk to the binding document for zx2967 family

ZTE's zx2967 I2S controller driver introduces pclk, this
patch documents this fact.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>