regulator: qcom: Refactor of-parsing code

Refactor out all custom property parsing code from the probe function
into a function suitable for regulator_desc->of_parse_cb usage.

Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Mark Brown <broonie@kernel.org>