Added support for ARMv7-M in arm io.

Added support for ARMv7-M targets in arm_nandwrite and
arm_nandread.

Change-Id: Iab1d78d401f735e191c6a8519f3619035a300fae
Signed-off-by: Henrik Nilsson <henrik.nilsson@bytequest.se>
Reviewed-on: http://openocd.zylin.com/1188
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>