drm/panfrost: Add support for multiple regulators

Some GPUs, namely, the bifrost/g72 part on MT8183, have a second
regulator for their SRAM, let's add support for that.

We extend the framework in a generic manner so that we could
support more than 2 regulators, if required.

Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviwed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200207052627.130118-5-drinkcat@chromium.org