msm: qpnp-power-on: program S3_TIMER register only based on DT property

Currently, S3 timer register is getting configured even when the
corresponding device tree property is not present. For e.g. S3
timer for secondary PMIC's PON is programmed to zero causing
immediate reset of that PMIC when the power + volume down keys
are pressed. Fix this by checking that the device tree property
is available before programming the timers.

CRs-Fixed: 783553
Change-Id: Ia78f7cd78b31930783bed51c63afea6edc775ac9
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>