imx6ul-14x14-evk has a wm8960 codec connected via SAI2 port.

Add support for it.

Thanks to Petr Kulhavy <brain@jikos.cz> for the hint on initializing
the PLL4 frequency to get a correct MCLK.
Signed-off-by: NFabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>