ASoC: wm_adsp: Add a handler for the compressed IRQ

Here support is added for responding to DSP IRQs that are used to
indicate data being available on the DSP. The idea is that we check the
amount of data available upon receipt of an IRQ and on subsequent calls
to the pointer callback we recheck once less than one fragment is
available (to avoid excessive SPI traffic), if there is truely less than
one fragment available we ack the last IRQ and wait for a new one.

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