karo: tx6: prevent DTB from either being out of reach of kernel or overwritten during...

The current setting of fdtaddr and fdt_high has proven to be
inappropriate in that either the FDT may be out of reach of the
kernel or may be overwritten by the uncompressing Linux kernel.

Set fdt_high to an offset of 256MiB from SDRAM start, so that it is
valid for any board regardless of memory size while preventing the
described problems.