ARM: vf610m4: add defconfig for Linux on Vybrids Cortex-M4

Add defconfig for Linux on Vybrid (vf610) on the secondary Cortex-
M4 CPU. The use of a XIP image has been tested which needs to be
loaded (e.g. using the custom m4boot loader) to the end of the
available RAM at address 0x8f000000. The Cortex-M4 has a code-alias
which makes sure that the instructions get fetched through the code
bus (alias starts at 0x00800000 => 0x80800000 in system address).
Hence, to get optimal performance, use 0x0f000000 as XIP_PHYS_ADDR.
This address is additionally shifted by the length of the minimal
loader which is inserted by m4boot. Currently, this offset is 0x80.

The standard DRAM base address is configured to 0x8C000000, which
gives the Cortex-M4 48MiB of RAM.

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