ASoC: Implement support for CLK_POUT as MCLK on Zylonite

The Zylonite supports switching the MCLK for the WM9713 between the
AC97CLK and CLK_POUT outputs of the PXA processor via switch SW15 on
the board. This patch adds support for configuring the system to use
CLK_POUT.

Unfortunately it is not possible to read the state of SW15 from software
so this feature is controlled by a module option 'clk_pout' which should
be set to a non-zero value to enable the use of CLK_POUT.

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