ASoC: wm_adsp: Move setting of DSP speed into CODEC specific code

The ADSP code should be agnostic of which CODEC it runs upon, currently
there is only one remaining part of the implementation that doesn't
follow this. When the DSP is booted on ADSP2 we read
ARIZONA_SYSTEM_CLOCK_1 and use that to set the initial speed for the DSP
clock. This patch factors that out into CODEC specific code, leaving the
ADSP code entirely CODEC agnostic.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>