target/arm: Document the architectural names of our GTIMERs

target/arm: Document the architectural names of our GTIMERs

As we are about to add more physical and virtual timers let's make it
clear what each timer does.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20250204125009.2281315-8-peter.maydell@linaro.org
[PMM: Add timer register name prefix to each comment]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>