ARM: dts: msm: reduce flash ramp rate for PMI8996

Currently, the flash_led device for PMI8994/8996 has a step up
and step down ramp rate of 6.7uS. For PMI8996, reduce it to 27uS
as per the systems team recommendation.

CRs-Fixed: 948392
Change-Id: I787916470a2ce6dda079259f60eda3ea50f0fa17
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>