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>