sepolicy: Define new policy rule to read gpu model

Add  a new file context label for gpu_model sysfs entry. allowed read
access to that entry.

Addressing the following denials :

type=1400 audit(0.0:628): avc: denied { read } for comm="RenderThread"
name="gpu_model" dev="sysfs" ino=26979
scontext=u:r:untrusted_app_25:s0:c512,c768 tcontext=u:object_r:sysfs_kgsl:s0
tclass=file permissive=0 app=com.snaptube.premium

Change-Id: I75afd661b8168227118bc5aa51952bee548fc506