ppc4xx: Remove 4xx NAND booting support

As ppc4xx currently only supports the deprecated nand_spl infrastructure
and nobody seems to have time / resources to port this over to the newer
SPL infrastructure, lets remove NAND booting completely.

This should not affect the "normal", non NAND-booting ppc4xx platforms
that are currently supported.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Tirumala Marri <tmarri@apm.com>
Cc: Matthias Fuchs <matthias.fuchs@esd.eu>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Tom Rini <trini@ti.com>
Tested-by: Matthias Fuchs <matthias.fuchs@esd.eu>