ARM: dts: msm: add sg-enable to tmc etr dt node for msm8939

Add sg-enable to TMC ETR DT node to indicate that scatter gather feature is
supported with TMC ETR configuration. sg-enable DT entry will help enable
this feature only on those targets which support it.

Change-Id: I8b2564b1d8fd9d3a8af5e2706773234274c08db7
Signed-off-by: Xiaogang Cui <xiaogang@codeaurora.org>