Add some tracepoints to the bcm2835_sdhost driver, to assist
debugging.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20180319161556.16446-2-peter.maydell@linaro.org