drm/amd/amdgpu: Move PWR_MISC_CNTL_STATUS to its own header

The register is part of the PWR block not the GC block.  Move to
its own header.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>