powerpc/p1022ds: set the clock-frequency prop only if the clock is enabled

The clock-frequency property in an audio codec's device tree node is set to
the input clock frequency for that codec.  On the Freescale P1022DS board,
the input clock is enabled only if the hwconfig 'audclk' option is set.
Therefore, the property should only be set in the device tree if the clock
is actually enabled.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>