opp: of: Allow empty opp-table with opp-shared

The opp binding now allows to have an empty opp table and shared-opp to
still describe that devices share v/f lines.

When initialising an empty opp table, allow such case by:
- treating such conditions with warnings in place of errors
- don't fail on empty table

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>