power: smb1351-charger: add support for configuring float voltage

Float voltage of the parallel charger also needs to be configured
if the float voltage of the main charger is adjusted. Add support
to configure the float voltage via POWER_SUPPLY_PROP_VOLTAGE_MAX
property.

Change-Id: Ifcf3b38e676cee9345615e134abe5593bc9535c0
Signed-off-by: Jie Cheng <rockiec@codeaurora.org>