[Why]
Logic & structures for generic regs does not exist in DC currently.

[How]
Implement register masks/shifts and relevant functions for generic mux,
similar to existing HPD and DDC objects.

V2: fix includes for kalloc/free (Alex)
Signed-off-by: Murton Liu <murton.liu@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Joshua Aberback <Joshua.Aberback@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>