ARM: dts: msm: Specify RGB_LED configuration for PM855L

RGB LEDs are present on PM855L which can be used for status
notifications (e.g. charging) on sdm855 platforms. They're
sourced by LPG/PWM channels.

Add the required configuration for it.

Change-Id: I5676e56855751e47a4a1882ee37eda61f666dfb5
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>