Cortex-M3: Add base support for Cortex-M3

This patch adds the base support for the Cortex-M3 processor (ARMv7-M
architecture). It consists of the corresponding arch/arm/mm/ files and
various #ifdef's around the kernel. Exception handling is implemented by
a subsequent patch.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>