iio: chemical: atlas-ph-sensor: switch regmap cache

switch from using REGCACHE_FLAT to REGCACHE_RBTREE so initial hw values
are read from device. This also allows some volatile ranges to be
dropped.

Note that REGCACHE_FLAT is intended only for very low lag cases so doesn't
do nice things like read initial values from the device.  Hence this change.

Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>