power: qpnp-smbcharger: adjust the float voltage of parallel charger

Currently, when the float voltage of the main charger is
modified through voltage_max property, float voltage of the
parallel charger is not being adjusted accordingly. Add support
to adjust the float voltage of parallel charger as well.

CRs-Fixed: 782125
Change-Id: Id99838551f32409876d025f03e00b6d04883627a
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>