msm: qpnp-pon: Add API to configure the PON trigger source

The API enables/disables power-on trigger capability of a PON source.
If a specific PON trigger is disabled it cannot act as a power-on
source to the PMIC.

CRs-Fixed: 489577
Change-Id: Ibec2063215eafc47701a3e255849e16f1d7b2cee
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>