drm/amd: Export DMCUB version to sysfs

For supported ASICs DMCU version is exported, but ASICs that support
DMCUB there is no information exported to sysfs.

Add an attribute for DMCUB.

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Link: https://lore.kernel.org/r/20250527155942.476354-1-mario.limonciello@amd.com


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