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>