ARM: introduce ARM_SINGLE_ARMV7M for ARMv7-M platforms

This introduces a new top level config symbol ARM_SINGLE_ARMV7M
for non-MMU, ARMv7-M platforms. It also support multiple ARMv7-M
platforms in one kernel image since the cores share the same
basic memory layout and interrupt controller. However, this works
only if the combined platforms also have a similar (main) memory
layout.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>