hw/watchdog/milkymist-sysctl.c: Switch to transaction-based ptimer API

hw/watchdog/milkymist-sysctl.c: Switch to transaction-based ptimer API

Switch the milkymist-sysctl code away from bottom-half based
ptimers to the new transaction-based ptimer API.  This just requires
adding begin/commit calls around the various places that modify the
ptimer state, and using the new ptimer_init() function to create the
timer.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20191021141040.11007-1-peter.maydell@linaro.org