power: qpnp-fg-gen4: Add qcom,fg-sys-min-voltage property

Add qcom,fg-sys-min-voltage property to specify the system
minimum voltage property so that it can be used to calculate the
available power.

Change-Id: I4d6872ee086ccd4972e68233bb5b54ec57342a76
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>