hw/timer/armv7m_systick: assert that board code set system_clock_scale

hw/timer/armv7m_systick: assert that board code set system_clock_scale

It is the responsibility of board code for an armv7m system to set
system_clock_scale appropriately for the CPU speed of the core.
If it forgets to do this, then QEMU will hang if the guest tries
to use the systick timer in the "tick at the CPU clock frequency" mode.

We forgot that in a couple of our boards (see commits ce4f70e81ed23c93f,
e7e5a9595ab1136). Add an assertion in the systick reset method so
we don't let any new boards in with the same bug.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20200825160847.18091-1-peter.maydell@linaro.org