Now that the CMSDK APB watchdog uses its Clock input, it will
correctly respond when the system clock frequency is changed using
the RCC register on in the Stellaris board system registers.  Test
that when the RCC register is written it causes the watchdog timer to
change speed.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210128114145.20536-22-peter.maydell@linaro.org
Message-id: 20210121190622.22000-22-peter.maydell@linaro.org