rockchip: Add an I2S driver

Add a driver for I2S which allows audio data to be sent from the SoC to
the audio codec. The sample rate and other settings are hard-coded for now
as there is no suitable device-tree binding available.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>