coresight: platform: acpi: Ignore the absence of graph

Some components may not have graph connections for describing
the trace path. e.g., ETE, where it could directly use the per
CPU TRBE. Ignore the absence of graph connections

Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>