dt-bindings: qpnp-fg-gen4: Add property to force calibration level

Add DT property to force the calibrate level. Once specified this
will ignore all the values and calibrate level is force set to this
level.

Change-Id: I88644b5e62966df4cb189a807d4d37c4b09796b6
Signed-off-by: Sahil Chandna <chandna@codeaurora.org>