drm/amd: Add missing kernel doc for prepare_suspend()

prepare_suspend() is intended to be used for any IP blocks
that must allocate memory during the suspend sequence.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/all/20231017143555.6a6450fc@canb.auug.org.au/
Fixes: cb11ca32 ("drm/amd: Add concept of running prepare_suspend() sequence for IP blocks")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/include/amd_shared.h

@@ -273,6 +273,8 @@ enum amd_dpm_forced_level;
  * @hw_init: sets up the hw state
  * @hw_fini: tears down the hw state
  * @late_fini: final cleanup
+ * @prepare_suspend: handle IP specific changes to prepare for suspend
+ *                   (such as allocating any required memory)
  * @suspend: handles IP specific hw/sw changes for suspend
  * @resume: handles IP specific hw/sw changes for resume
  * @is_idle: returns current IP block idle status