arm: update processor types

Add the Cortex M3 variant. These microcontrollers don't support regular
ARM instructions and don't have an MMU.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>