ASoC: ak4613: Implement suspend callback

Add the suspend callback to accompany the existing resume operation.
With the suspend/resume callbacks the regmap (regcache) state handling
can follow the recommended sequence.

Based on commit a2ebd586 ("ASoC: ak4642: Implement suspend
callback") by Peter Ujfalusi <peter.ujfalusi@ti.com>.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>