Adds driver support to preserve DSU PMU register values over a DSU
power cycle. This driver needs to be enabled by the platforms that
support DSU and also need it's PMU registers to be preserved
Change-Id: I7fc68a3d7d99ee369379aa5cd114fffc763fc0d2
Signed-off-by: Arvind Ram Prakash <arvind.ramprakash@arm.com>