soc: arm: remove default selection of system timer for ARM platforms

We shall not enable by default a system timer in ARM
platforms, namely the SysTick, the Nordic, or the SAM0
RTC timer, simply by assessing the hardware capabilities
(e.g. by conditioning on CPU_CORTEX_M_HAS_SYSTICK).
Instead, now, all ARM platforms needs to explicitly set
their system timer module. Note that this has already
been the case for ca 80% of the ARM platforms.

This clean-up allows us to decouple HW capabilities from
system configuration (for example, Nordic platforms may
enable option CPU_CORTEX_M_HAS_SYSTICK, and still use
the platform-specific RTC timer for system timing).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>