power: axp: merge separate DLDO functions into 1

Instead of one function for each DLDO regulator, make 1 function that
takes an extra "index". Since the control bits for the DLDO regulators
are contiguous, this makes the function very simple. This removes a lot
of duplicate code.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>