arch/arm: add Cortex-M4 entry

arch/arm: add Cortex-M4 entry

This commit adds the option to select the Cortex-M4 ARM core, in the
same family as Cortex-M3. This will be useful to enable the internal
toolchain backend for this ARM core, and provide some defconfigs for
Cortex-M4 platforms.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>