drm/imagination: Document pvr_device.power member

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

Fixes: 330e76d ("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>

Author: MTCoster

drivers/gpu/drm/imagination/pvr_device.h

@@ -146,6 +146,14 @@ struct pvr_device {
 	 */
 	struct clk *mem_clk;
 
+	/**
+	 * @power: Optional power domain devices.
+	 *
+	 * On platforms with more than one power domain for the GPU, they are
+	 * stored here in @domain_devs, along with links between them in
+	 * @domain_links. The size of @domain_devs is given by @domain_count,
+	 * while the size of @domain_links is (2 * @domain_count) - 1.
+	 */
 	struct pvr_device_power {
 		struct device **domain_devs;
 		struct device_link **domain_links;