power: qpnp-smbcharger: support enable/disable safety timers

Currently, pre-charge and total charge safety timers are enabled
by default if the safety timer timeouts are configured. Add support
to disable/enable them through POWER_SUPPLY_PROP_SAFETY_TIMER_ENABLE
property.

CRs-Fixed: 773772
Change-Id: Iff7d66b3bf391c2c7b5bdabfdf2147d3fce9065d
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>