ASoC: arizona: Allow runtime reconfiguration of the output mode

Some systems use external analogue switches to connect more analogue
devices to the CODEC than are supported by the device.  In some systems
this requires changing the switched output from single ended to
differential mode dynamically at runtime. Add a new function
arizona_set_output_mode() to support this.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>