ARM: dts: msm: Add cluster node in lpm levels

Add dummy cluster node under lpm levels to make it work
with default lpm levels frame work. Also rename gdhs
mode with retention.

Change-Id: I1bc3b0bf88e68ff241a5ba338da691797f563d26
Signed-off-by: Achyuth Sai Vadrav <avadra@codeaurora.org>