msm: qpnp-power-on: add support for configuring SMPL PON trigger

Add a sysfs node named "smpl_en" to allow configuring the SMPL PON
trigger. This sysfs node takes the values 0 or 1 for disable or enable
respectively.

This example shell command will disable the SMPL PON trigger.
echo 0 > /sys/module/qpnp_power_on/parameters/smpl_en

CRs-Fixed: 957185
Change-Id: I32ebe84ceaac21443b28abea2b4935a96d94c9cd
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>