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

BugLink: http://bugs.launchpad.net/bugs/1590455

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

sound/soc/codecs/ak4642.c

@@ -560,6 +560,7 @@ static const struct regmap_config ak4642_regmap = {
 	.max_register		= FIL1_3,
 	.reg_defaults		= ak4642_reg,
 	.num_reg_defaults	= NUM_AK4642_REG_DEFAULTS,
+	.cache_type		= REGCACHE_RBTREE,
 };
 
 static const struct regmap_config ak4643_regmap = {
@@ -568,6 +569,7 @@ static const struct regmap_config ak4643_regmap = {
 	.max_register		= SPK_MS,
 	.reg_defaults		= ak4643_reg,
 	.num_reg_defaults	= ARRAY_SIZE(ak4643_reg),
+	.cache_type		= REGCACHE_RBTREE,
 };
 
 static const struct regmap_config ak4648_regmap = {
@@ -576,6 +578,7 @@ static const struct regmap_config ak4648_regmap = {
 	.max_register		= EQ_FBEQE,
 	.reg_defaults		= ak4648_reg,
 	.num_reg_defaults	= ARRAY_SIZE(ak4648_reg),
+	.cache_type		= REGCACHE_RBTREE,
 };
 
 static const struct ak4642_drvdata ak4642_drvdata = {