drm/amdgpu: add osssys v4_2 ip headers (v2)

v1: add osssys v4_2 register offset and shift masks
header files. vega20 and arcturus will refer to
these ip headers. (Hawking)
v2: clean up osssys v4_2 registers (Alex)

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Dennis Li <Dennis.Li@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>