qpnp-fg-gen3: export the battery float voltage

Implement VOLTAGE_MAX_DESIGN to expose the battery float voltage. This
property will reflect the values read from battery profile and will
will be used for setting the max voltage on charging chips.

Change-Id: I4b9028f8930ac2f3de6c043fa145494f37322160
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>