powerpc/sequoia: Fix NAND partitions not to overlap

Currently the DTS defines two partitions at the same addresses, if you
use one, you will corrupt information on the other one. Fix it by
shifting the second partition.

Signed-off-by: Pavel Machek <pavel@denx.de>
[mpe: Reconstruct change log from email thread]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>