powerpc: ppc4xx: remove lwmon5 support

This has not been converted to Generic Board, so should be removed.
(See doc/README.generic-board for details.)

Remove CONFIG_LWMON5 references.
(Also, remove undefined CONFIG_WD_MAX_RATE while I am here.)

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Stefan Roese <sr@denx.de>