lib: cpu: Check SCU presence in DSU before accessing DSU registers

lib: cpu: Check SCU presence in DSU before accessing DSU registers

The DSU contains system control registers in the SCU and L3 logic to
control the functionality of the cluster. If "DIRECT CONNECT" L3
memory system variant is used, there won't be any L3 cache,
snoop filter, and SCU logic present hence no system control register
will be present. Hence check SCU presence before accessing DSU register
for DSU_936184 errata.

Signed-off-by: Pramod Kumar <pramod.kumar@broadcom.com>
Change-Id: I1ffa8afb0447ae3bd1032c9dd678d68021fe5a63