drm/amdgpu: Add PSP loading support for DMCUB ucode

DMCUB ucode requires secure loading through PSP. This is already
supported in PSP as GFX_FW_TYPE_DMUB, it just needs to be mapped from
AMDGPU_UCODE_ID_DMCUB to GFX_FW_TYPE_DMUB.

DMUB is a shorthand name for DMCUB and can be used interchangeably.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>