ASoC: rockchip-i2s: enable "hclk" for rockchip I2S controller

As "hclk" is used for rockchip I2S controller, driver must to enable
it in probe.

Tested on RK3288 with max98090.

Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>