drm/panfrost: add regulators to devfreq

Some OPP tables specify voltage for each frequency. Devfreq can
handle these regulators but they should be get only 1 time to avoid
issue and know who is in charge.

If OPP table is probe don't init regulator.

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200710095409.407087-11-peron.clem@gmail.com