[POWERPC] cell: CONFIG_SPE_BASE is a typo

The config symbol for SPE support is called CONFIG_SPU_BASE, not
CONFIG_SPE_BASE.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>