configs: vf610: increase maximum size and enforce correct limit

On Vybrid SoCs U-Boot gets loaded into GFX SRAM which is 512KiB.
Currently 32KiB is reserved for the IMX header. However, this is
not reflected in the size limit. In v2017.11-rc2 the actual size
limit (512KiB-32KiB) has been reached for Colibri VF61, which
lead to a successful build of U-Boot but not a working binary.

The IMX header is much smaller than 32KiB, typically around 1KiB.
Decrease the reserved size to 4KiB and specify the correct U-Boot
size limit. Apply this new base address and limit for all Vybrid
based boards.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>