ASoC: ak4642: Enable cache usage to fix crashes on resume

The ak4642 driver is using a regmap cache sync to restore the
configuration of the chip on resume but (as Peter observed) does not
actually define a register cache which means that the resume is never
going to work and we trigger asserts in regmap.  Fix this by enabling
caching.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reported-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org