ARM: dts: msm: Set number of LEDs per string for WLED for SM8150 platforms

For SM8150 platforms, the display panel widely used on CDP and
MTP are having 6 LEDs per string. Specify them so that the
WLED flash current can be calculated based on the battery
parameters, number of strings and number of LEDs per string.

Change-Id: I260c21430019fb3f9f79b5a93ac13a5a8b90c2e2
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>