coresight: etm4x: Add commentary on the registers

coresight: etm4x: Add commentary on the registers

As we are about define a switch..case table for individual register
access by offset for implementing the system instruction support,
document the possible set of registers for each group to make
it easier to correlate.

Link: https://lore.kernel.org/r/20210110224850.1880240-11-suzuki.poulose@arm.com
Cc: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20210201181351.1475223-13-mathieu.poirier@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>