clk: zte: add i2s clocks for zx296718

The i2s related clock support is missing from the existing zx296718
clock driver. This patch adds it, so that the upstream ZX I2S driver
can work out.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>