Add CORTEX_M_TIMER Kconfig option

Adds the Kconfig option for selecting the Cortex-M systick timer.  This helps
pave the way for moving the Cortex-M systick timer into the drivers/ directory.

Change-Id: Iee040083a425c9c1dbc5973991b89239e0a72eee
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>