drm/amd: Add DM DMCU support

DMCU (Display Microcontroller Unit) is a GPU chip involved in
eDP features like Adaptive Backlight Modulation and Panel Self
Refresh.

DC is already fully equipped to initialize DMCU as long as the
firmware is loaded.

At the moment only the raven firmware is available.

A single .bin file is loaded by the kernel's loading mechanism
and split into two ucodes according to the header.

DMCU is optional, so if the firmware is not found, no error or
warning is raised.

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>