mpc85xx: enable NAND support for all subtargets

In subtarget p2020, there wasn't enabled nand support, and because of
that there weren't available tools from mtd-utils package, which has
utilities for NAND flash memory even though reference board, which
is the only currently supported device in p2020 subtarget has NAND [1].

All subtargets in mpc85xx has already enabled nand support, let's do it
globally.

[1] https://www.nxp.com/design/qoriq-developer-resources/p2020-reference-design-board:P2020RDB



Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>
(cherry picked from commit 6006f733)