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

hw/timer/cmsdk-apb-timer: 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 pclk-frq
property to using the Clock once all the users of this device have
been converted to wire up the Clock.

Since the device doesn't already have a doc comment for its "QEMU
interface", we add one including the new Clock.

This is a migration compatibility break for machines mps2-an505,
mps2-an521, musca-a, musca-b1.

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: 20210128114145.20536-8-peter.maydell@linaro.org
Message-id: 20210121190622.22000-8-peter.maydell@linaro.org