hw/watchdog/cmsdk-apb-watchdog: Add Clock input

As the first step in converting the CMSDK_APB_TIMER device to the
Clock framework, add a Clock input.  For the moment we do nothing
with this clock; we will change the behaviour from using the
wdogclk-frq property to using the Clock once all the users of this
device have been converted to wire up the Clock.

This is a migration compatibility break for machines mps2-an385,
mps2-an386, mps2-an500, mps2-an511, mps2-an505, mps2-an521, musca-a,
musca-b1, lm3s811evb, lm3s6965evb.

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: 20210128114145.20536-10-peter.maydell@linaro.org
Message-id: 20210121190622.22000-10-peter.maydell@linaro.org