ARM: ARMv7-M: Allow the building of new kernel port

This patch modifies the required Kconfig and Makefile files to allow the
building of kernel for Cortex-M3.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Jonathan Austin <jonathan.austin@arm.com>
Tested-by: Jonathan Austin <jonathan.austin@arm.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>