powerpc: lwmon5: remove redundant CONFIG_SPL_* defines

The CPU directory of this board is arch/powerpc/cpu/ppc4xx.
Without the CONFIG_SPL_START_S_PATH and CONFIG_SPL_LDSCRIPT defines,
the same start.o and u-boot-spl.lds are selected by default.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Wolfgang Denk <wd@denx.de>
Acked-by: Stefan Roese <sr@denx.de>