power: qpnp-charger: allow vinmin to be lowered to 4.0 V

Currently the minimum input voltage is limited to a minimum
value of 4.2 V. Some chargers in combination with cables
having high resistance may require the input voltage minum
to be lower than 4.2.

Fix this by allowing up to 4.0 V for the minimum input voltage
settings.

Change-Id: I6f1dfa2453c7b95ecc2ccdf2aac23981c0e4dd0a
CRs-Fixed: 584999
Signed-off-by: David Keitel <dkeitel@codeaurora.org>