coresight: Add a helper to check if a device is source

Since there are a lot of places in the code to check whether the
device is source, add a helper to check it.

Signed-off-by: Tao Zhang <quic_taozha@quicinc.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20241213100731.25914-3-quic_taozha@quicinc.com