dt-bindings: opp: Describe H616 OPPs and opp-supported-hw

Compared to the existing Allwinner H6 OPP scheme, the H616 uses a
similar NVMEM based mechanism to determine the silicon revision, which
is required to select the right frequency / voltage pair for the OPPs.
However it limits the maximum frequency for some speed bins, also seems
to not support all frequencies in all speed bins, which requires us to
introduce the opp-supported-hw property.

Add this property to the list of allowed properties, also drop the
requirement for the revision specific opp-microvolt properties, since
they might not be needed if using opp-supported-hw.

Also use to opportunity to adjust some wording, and drop a sentence
referring to the Linux driver and the OPP subsystem.

Shorten the existing example and add another example, showcasing the
opp-supported-hw property.

Signed-off-by: Martin Botka <martin.botka@somainline.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>