ARM: dts: imx6ul-14x14-evk: Add audio support

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: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>