board: tbs2910: Use recommended distroboot addresses

According to doc/README.distro fdt_addr must not be set when DTB is not
available from hardware. So remove this entry.

Use address 32MB above the start of DRAM for kernel_addr_r. This way
we likely can avoid the self-relocation of the compressed kernel image
before it decompresses to offset 0x8000 from start of DRAM.

Use address 128MB above the start of DRAM for fdt_addr_r, since this is
the maximum location for the end of the kernel. So we avoid overwriting
the DTB.

Use 512k above that for ramdisk_addr_r. This should be enough for the
DTB, rest of DRAM can be used for initrd.

Place boot script / extlinux.conf at offset 0 / 1MB from start of DRAM.
This space is available for processing in U-Boot.

Signed-off-by: Soeren Moch <smoch@web.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>