sandbox: trace: Increase trace buffer size

When running the trace test on the sandbox platform, the current size
of 16MiB is no longer large enough for capturing the entire trace
history, and results in truncation. Use a size of 32MiB for the trace
buffer on the sandbox platform while running the trace test.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>