regulator: mt6311: Use REGCACHE_RBTREE

This regulator is on a slow i2c bus.  Register accesses are very simple,
they all either enable/disable a regulator channel, or select a new
voltage level.  Thus, reading registers from the device will always
return what was last written.

Therefore we can save a lot of time when reading registers by using a
regmap_cache.  Since the register map is relatively large, but we only
ever access a few of them, we use an RBTREE cache.
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Henry Chen <henryc.chen@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/mt6311-regulator.c

@@ -30,6 +30,7 @@ static const struct regmap_config mt6311_regmap_config = {
 	.reg_bits = 8,
 	.val_bits = 8,
 	.max_register = MT6311_FQMTR_CON4,
+	.cache_type = REGCACHE_RBTREE,
 };
 
 /* Default limits measured in millivolts and milliamps */