coresight: tmc: Make memory width mask computation into a function

Make the computation of a memory mask representing the width of the memory
bus into a function so that it can be re-used by the ETR driver.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Leo Yan <leo.yan@linaro.org>
Link: https://lore.kernel.org/r/20190829202842.580-16-mathieu.poirier@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>