Implement set_sysclk() and then rather than assuming 256fs use the
supplied value to calculate and configure the clock ratio for the
currently used sample rate. As a side effect we also end up
implementing clock selection for the ADC path.

In order to avoid confusion remove the existing set_clkdiv() based
configuration of the clock source for the DAC and update the SMDK64xx
driver (which is the only in-tree user of the CODEC).
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>