Connect up the armv7m clocks on the mps2-an385/386/500/511.

Connect up the armv7m object's clocks on the MPS boards defined in
mps2.c.  The documentation for these FPGA images doesn't specify what
systick reference clock is used (if any), so for the moment we
provide a 1MHz refclock, which will result in no behavioural change
from the current hardwired 1MHz clock implemented in
armv7m_systick.c:systick_scale().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Message-id: 20210812093356.1946-9-peter.maydell@linaro.org