sepolicy: Add property context for vendor GPU frequencies

Added property vendor.gpu.available_frequencies and
rule to allow init_shell to set this property.It resolves the
below avc denial.

selinux: avc: denied { set } for property=ro.gpu.available_frequencies
pid=686 uid=0 gid=0 scontext=u:r:qti_init_shell:s0
tcontext=u:object_r:default_prop:s0 tclass=property_service
permissive=0

CRs-Fixed: 2283789
Change-Id: I102a33cbfd7f2f538751f98e88ae0e434e94cea5