cortex_m: declare local functions as static

cortex_m: declare local functions as static

Functions that are not used outside the file should be declared as
static.

Change-Id: Ie81f6bdce91e2a1456364b47f30aa4d35c7ee7bc
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5900
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Tested-by: jenkins