coresight: fixing CPU hwid lookup in device tree

Some DT specification will represent CPU nodes with address
cells greater than one, making the current code fail.  Using
the proper retrieval helper function ensure the correct hwid
for CPUs is read properly with different address cell size.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>