dt-bindings: marvell,mmp2: Add ids for the power domains

On MMP2 the audio and GPU blocks are on separate power islands. On MMP3
the camera block's power is also controlled separately.

Add the numbers that we could use to refer to the power domains for
respective power islands from the device tree.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lkml.kernel.org/r/20200519224151.2074597-11-lkundrak@v3.sk
Signed-off-by: Stephen Boyd <sboyd@kernel.org>