uboot-lantiq: danube: fix SPL boot

uboot-lantiq: danube: fix SPL boot

On danube we only have 0x6800 bytes of usable SRAM. Everything behind
can't be written to and a SPL u-boot locks up during boot.

Since it's a hard to debug issue and took me more than two years to fix
it, I consider it worth to include fix albeit SPL u-boots are not build
in OpenWrt.

I faced the issue while trying to shrink the u-boot to 64K since some
boards only have an u-boot partition of that size from the days
ifx-uboot was used.

Signed-off-by: Mathias Kresin <dev@kresin.me>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>