ASoC: wm8940: Enable cache usage to fix crashes on resume

The wm8940 driver is using a regmap cache sync to restore the
configuration of the chip when switching from OFF to STANDBY, but does
not actually define a register cache which means that the restore is
never going to work and we trigger asserts in regmap.  Fix this by
enabling caching.

Based on commit d3030d11961a8c10 ("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>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/wm8940.c

@@ -743,6 +743,7 @@ static const struct regmap_config wm8940_regmap = {
 	.max_register = WM8940_MONOMIX,
 	.reg_defaults = wm8940_reg_defaults,
 	.num_reg_defaults = ARRAY_SIZE(wm8940_reg_defaults),
+	.cache_type = REGCACHE_RBTREE,
 
 	.readable_reg = wm8940_readable_register,
 	.volatile_reg = wm8940_volatile_register,