soc: qcom: qpnp-pbs: add an API to trigger single PBS event

A simple PBS can be triggered by SW writing to SW_TRIGGER (bit 7) in
TRIG_CTL register without passing any parameters through SCRATCH1
register and not waiting on any condition. Also, this simple PBS is more
like a fire and forget and doesn't even need to check on the status
through SCRATCH2 register. Add support for this simple PBS through an
API qpnp_pbs_trigger_single_event().

Change-Id: I8a6448f9be8f84269c6ba5672bead01a152185ad
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>