power: qpnp-charger: update VBAT_WEAK register in qpnp charger probe

Currently weak battery voltage threshold(VBAT_WEAK) is getting
set in bootloaders and QPNP charger driver is not modifying
VBAT_WEAK during the probe. So set the VBAT_WEAK register
explicitly in the qpnp charger probe function, if a value
is specified for it in the charger device tree node.

CRs-Fixed: 520770
Change-Id: I054e9c2bf14a666f896faf3b4c8d2a6e2201b5fb
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>