ARM: dts: imx6q-cm-fx6: add analog audio support

The cm-fx6 module has an onboard Wolfson wm8731 codec which is muxed
to the ssi2 controller. Unlike most (all?) supported i.MX6 board/codec
combinations the wm8731 is operated in slave mode and the clock setup
is static.

Add support for it.

Signed-off-by: Valentin Raevsky <valentin@compulab.co.il>
[christopher.spinrath@rwth-aachen.de: enhanced commit message, ported
  to upstream and some cleanup]
Signed-off-by: Christopher Spinrath <christopher.spinrath@rwth-aachen.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>