leds: qpnp-wled: Add support to configure auto PFM for pmicobalt

The WLED boost controller in pmicobalt supports auto PFM mode when
LCD mode is selected. Auto PFM mode is enabled by default except
on some specific hardware versions. Add a property to specify
AUTO_PFM_THRESHOLD for the boost controller so that it can enter
PFM mode when the adaptive headroom voltage falls below the
threshold.

CRs-Fixed: 1081738
Change-Id: I964b3452d0cdb3618b4ab446655ae75fa3a1049d
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>