power: smb135x-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.

CRs-Fixed: 782125
Change-Id: Ib83aec8466bea8cbd0556d169c822b8738824329
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>