ppc4xx: Remove 4xx NAND booting support

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 <[email protected]>
Cc: Wolfgang Denk <[email protected]>
Cc: Tirumala Marri <[email protected]>
Cc: Matthias Fuchs <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Tom Rini <[email protected]>
Tested-by: Matthias Fuchs <[email protected]>