ASoC: ak4613: Enable cache usage to fix crashes on resume

During system resume:

    kernel BUG at drivers/base/regmap/regcache.c:347!
    ...
    PC is at regcache_sync+0x1c/0x128
    LR is at ak4613_resume+0x28/0x34

The ak4613 driver is using a regmap cache sync to restore the
configuration of the chip on resume but 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.

Based on commit d3030d11

 ("ASoC: ak4642: Enable cache usage to
fix crashes on resume") by Mark Brown <broonie@kernel.org>.

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