iio:ad5064: Use a enum for the register map layout type

Currently the ad5064 only supports two different register map variations
and this is represented by a bool. This patch changes since to a enum so
we can support more variations in the future.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>