[Why]
DCE6 has no SMU_INTERRUPT_CONTROL register, but it's used for DCN10 and later

[How]
Add DCE6 specific macros definitions for DMCU registers and masks
DCE6 DMCU macros will avoid buiding errors when using DCE6 headers
There is no other change needed in dce_dcmu
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>