Wire up the sysclk input to the armv7m object.

Strictly this SoC should not have a systick device at all, but our
armv7m container object doesn't currently support disabling the
systick device.  For the moment, add a TODO comment, but note that
this is why we aren't wiring up a refclk (no need for one).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alexandre Iooss <erdnaxe@crans.org>
Message-id: 20210812093356.1946-16-peter.maydell@linaro.org