dt-bindings: 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: I37f684974ad5a6e4ce6043b72d6a1cfa91ca84ac
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>