clk: mmp2: Add support for power islands

Apart from the clocks and resets, the PMU hardware also controls power
to peripherals that are on separate power islands. On MMP2, that's the
GC860 GPU and the SSPA audio interface, while on MMP3 also the camera
interface is on a separate island, along with the pair of GC2000 and GC300
GPUs and the SSPA.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lkml.kernel.org/r/20200519224151.2074597-12-lkundrak@v3.sk
Signed-off-by: Stephen Boyd <sboyd@kernel.org>