power: bcl: Add frequency mitigation as an optional property for BCL

For targets with LMH DCVSh hardware and OSM, BCL software frequency
mitigation is not required. Since hardware is doing the frequency
mitigation, there is no need for the HLOS BCL frequency mitigation.
So make the properties "qcom,mitigation-freq-khz" and
"qcom,thermal-handle" as optional properties.

Change-Id: I0062f3b39f00ff2f0e74affcffbcf1afd89d3b2f
Signed-off-by: Manaf Meethalavalappu Pallikunhi <manafm@codeaurora.org>