hw/timer/armv7m_systick: Update clock source before enabling timer

hw/timer/armv7m_systick: Update clock source before enabling timer

Starting the SysTick timer and changing the clock source a the same time
will result in an error, if the previous clock period was zero. For exmaple,
on the mps2-tz platforms, no refclk is present. Right after reset, the
configured ptimer period is zero, and trying to enabling it will turn it off
right away. E.g., code running on the platform setting

    SysTick->CTRL  = SysTick_CTRL_CLKSOURCE_Msk | SysTick_CTRL_ENABLE_Msk;

should change the clock source and enable the timer on real hardware, but
resulted in an error in qemu.

Signed-off-by: Richard Petri <git@rpls.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20220201192650.289584-1-git@rpls.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>