hw/timer/armv7m_systick: Use clock inputs instead of system_clock_scale

hw/timer/armv7m_systick: Use clock inputs instead of system_clock_scale

Now that all users of the systick devices wire up the clock inputs,
use those instead of the system_clock_scale and the hardwired 1MHz
value for the reference clock.

This will fix various board models where we were incorrectly
providing a 1MHz reference clock instead of some other value or
instead of providing no reference clock at all.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Damien Hedde <damien.hedde@greensocs.com>
Message-id: 20210812093356.1946-22-peter.maydell@linaro.org