nand: Remove unused ppc4xx NAND driver and references

ppc4xx support was removed some time ago. Lets remove the now unused
NAND driver and all its references for this platform as well.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Scott Wood <oss@buserror.net>