arm64/debug: Make use of the system wide safe value

Use the system wide value of ID_AA64DFR0 to make safer decisions

Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Tested-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>