clk: rockchip: rk3368: enable the CLK_SET_RATE_PARENT flag for i2s_2ch

I2S_2CH set freq need to select parent and calculate parent freq.
so just mark it as the CLK_SET_RATE_PARENT flag.

Signed-off-by: zhangqing <zhangqing@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>