system/dma-helpers.c: Move trace events to system/trace-events

system/dma-helpers.c: Move trace events to system/trace-events

The dma-helpers.c file is in the system/ subdirectory, but it
defines its trace events in the root trace-events file. Move
them to the system/trace-events file where they more naturally
belong.

Fixes: 800d4deda0 ("softmmu: move more files to softmmu/")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20241108162909.4080314-2-peter.maydell@linaro.org