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

commit d3030d11 upstream.

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>
[ kamal: backport to 4.2-stable: no separate ak4643_regmap ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

sound/soc/codecs/ak4642.c

@@ -508,6 +508,7 @@ static const struct regmap_config ak4642_regmap = {
 	.max_register		= ARRAY_SIZE(ak4642_reg) + 1,
 	.reg_defaults		= ak4642_reg,
 	.num_reg_defaults	= ARRAY_SIZE(ak4642_reg),
+	.cache_type		= REGCACHE_RBTREE,
 };
 
 static const struct regmap_config ak4648_regmap = {
@@ -516,6 +517,7 @@ static const struct regmap_config ak4648_regmap = {
 	.max_register		= ARRAY_SIZE(ak4648_reg) + 1,
 	.reg_defaults		= ak4648_reg,
 	.num_reg_defaults	= ARRAY_SIZE(ak4648_reg),
+	.cache_type		= REGCACHE_RBTREE,
 };
 
 static const struct ak4642_drvdata ak4642_drvdata = {