drm/amdgpu: Add reg_state sysfs attribute

Add reg_state attribute to fetch the register snapshot of different
IPs like XGMI, WAFL,PCIE and USR. To get a snapshot for a particular IP
	1) Open the sysfs file
	2) Seek to the offset as defined in amdgpu_sysfs_reg_offset
	3) Read

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>