clk: rockchip: rk3288 export i2s0_clkout for use in DT

This exposes the clock that comes out of the i2s block which generally
goes to the audio codec.

Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
[removed CLK_SET_RATE_PARENT from original patch]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>