powerpc: Correct dependency of KEXEC

commit 28794d34

 ("powerpc/kconfig: Kill
PPC_MULTIPLATFORM") broke KEXEC, by making it dependent on BOOK3S, while it
should be PPC_BOOK3S.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>