ASoC: McASP: make AHCLK direction configurable

Add a .set_sysclk function to pass the direction of the clock down to
the driver. Only enable AHCLKX in the PDIR register when the CPU is
driving the clock.

This also removes the modification of the AHCLKXE/AHCLKRE bits in the
hw_params callback, and users must set the desired configuration using
snd_soc_dai_set_sysclk(), which this patch also does for the only user
in mainline (davinci-evm).

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>