regulator: act8865: Convert to regulator core's simplified DT parsing code

Use regulator core's simplified DT parsing code to simply the driver
implementation. With this conversion, also move the NULL test against pdata
in act8865_get_regulator_data() to the caller. This makes it clear the
code path to explicitly set init_data is for non-DT case only.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>