arch: arm: cortex_m: conditionally select ARCH_HAS_TIMING_FUNCTIONS

Cortex-M SoCs implement (optionally) the Data Watchpoint and
Tracing Unit (DWT), which can be used for timing functions.
Select the corresponding ARCH capability if the SoC implements
the DWT.

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