mfd: arizona: Split INx_MODE into two fields

Later arizona silicon has the single/differential selector
in a different register, and IN1_MODE only selects between
analogue or digital. Prepare for this by splitting the
INx_MODE definition into two fields.

Signed-off-by: Richard Fitzgerald <rf@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>