ARM: vf610: enable Cortex-M4 configuration on Vybrid SoC

This patch allows to build the Kernel for Vybrid (VF6xx) SoC
when ARMv7-M CPU is selected. The resulting image runs on the
secondary Cortex-M4 core. This core has equally access to all
peripherals as the main Cortex-A5 core. However, there is no
resource control mechanism, hence when both cores are used
simultaneously, orthogonal device tree's are required.

The boot CPU is dependent on the SoC variant. The available
boards use mostly variants where the Cortex-A5 is the primary
and hence the boot CPU. Booting the secondary Cortex-M4 CPU
needs SoC specific registers written. There is no in kernel
support for this right now, a external userspace utility
called "m4boot" can be used to boot the kernel:

m4boot xipImage initramfs.cpio.lzo vf610m4-colibri.dtb

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