drm/amd: Add documentation for how to flash a dGPU

The flashing process for dGPUs uses sysfs files in a
non-obvious way, so document it for users.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>