regulator: ti-abb-regulator: do not open-code counting and access of dt array elements

Open coding the counting of elements in a dt-property is abstracted by the newly
introduced of_property_count_uXX_elems functions. Additionally the raw iteration
over the states element exposes the endian conversion and dtb-format details,
which according to Mark Rutland "would be nice to limit [...] to of_ helper
functions".

Thus change ti-abb-regulator to use the helper for element counting and
of_property_read_u32_index for retrieval of individual values.

This makes it possible to remove the raw access to the property entirely.

Signed-off-by: Heiko Stuebner <heiko.stuebner@bqreaders.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Mark Brown <broonie@linaro.org>