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 d3030d1196 ("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>

sound/soc/codecs/ak4613.c

@@ -146,6 +146,7 @@ static const struct regmap_config ak4613_regmap_cfg = {
 	.max_register		= 0x16,
 	.reg_defaults		= ak4613_reg,
 	.num_reg_defaults	= ARRAY_SIZE(ak4613_reg),
+	.cache_type		= REGCACHE_RBTREE,
 };
 
 static const struct of_device_id ak4613_of_match[] = {