drm/imagination: Add power domain control

drm/imagination: Add power domain control

The first supported GPU only used a single power domain so this was
automatically handled by the device runtime.

In order to support multiple power domains, they must be enumerated from
devicetree and linked to both the GPU device and each other to ensure
correct power sequencing at start time.

For all Imagination Rogue GPUs, power domains are named "a", "b", etc. and
the sequence A->B->... is always valid for startup with the reverse true
for shutdown. Note this is not always the *only* valid sequence, but it's
simple and does not require special-casing for different GPU power
topologies.

Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Link: https://lore.kernel.org/r/20250410-sets-bxs-4-64-patch-v1-v6-5-eda620c5865f@imgtec.com
Signed-off-by: Matt Coster <matt.coster@imgtec.com>