armv7m: Add SysTick timer driver

The SysTick is a 24-bit down counter that is found on all ARM Cortex
M3, M4, M7 devices and is always located at a fixed address.

The number of reference clock ticks that correspond to 10ms is normally
defined in the SysTick Calibration register's TENMS field. However, on some
devices this is wrong, so this driver allows the clock rate to be defined
using CONFIG_SYS_HZ_CLOCK.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Reviewed-by: Vikas MANOCHA <vikas.manocha@st.com>