UPSTREAM: coresight: Cleanup device subtype struct

Clean up our struct a little bit by using a union instead of
a struct for tracking the subtype of a device.

Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

(Upstream commit 00b78e8b7b003c686a8570df7d231787f514226c).

Bug: 140266694
Change-Id: I7cfbc6d8a2f0552e1361e04ce06b4bbb277448e6
Signed-off-by: Yabin Cui <yabinc@google.com>