dt-bindings: opp: Allow empty OPP tables

Currently the optional property opp-shared is used within an opp table
to tell that a set of devices share their clock/voltage lines (and the
OPP points).
It is therefore possible to use an empty OPP table to convey only that
information, useful in situations where the opp points are provided via
other means (hardware. firmware, etc).

Update the documentation to remark this additional case and provide an
example.

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