OPP: Simplify opp_parse_supplies() by restructuring it

opp_parse_supplies() has grown into too big of a routine (~190 lines)
and it is not straight-forward to understand it anymore.

Break it into smaller routines and reduce code redundancy a bit by using
the same code to parse properties.

This shouldn't result in any logical changes.

Tested-by: James Calligeros <jcalligeros99@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>