test/benchmarks/latency_measure: adapt test to 24bit Systick for STM32

Cortex-M has 24bit systick.
But this test by default set 1 TICK per seconds, which  is
achievable only if frequency is below 0x00FFFFFF (around 16MHz).
20 Ticks per secondes allows a frequency up to 335544300Hz (335MHz)

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@st.com>