pwm: pwm-qti-lpg: Add property for LPG channel synchronization

Add qcom,sync-channel-ids property for grouping the LPG channels which
are expected to be enabled with a synchronized pattern.

Change-Id: I1f5d83f2e6094771cb30d567d9d4d4476f1f3dc9
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>