drm/imagination: Document pvr_device.power member

drm/imagination: Document pvr_device.power member

Automated testing caught this missing doc comment; add something suitable
(and useful).

Fixes: 330e76d31697 ("drm/imagination: Add power domain control")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/r/20251106152448.453b53ad@canb.auug.org.au/
Reviewed-by: Alessio Belle <alessio.belle@imgtec.com>
Link: https://patch.msgid.link/20251121-device-power-doc-fix-v2-1-3417779f36c7@imgtec.com
Signed-off-by: Matt Coster <matt.coster@imgtec.com>