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>

arch/powerpc/Kconfig

@@ -316,7 +316,7 @@ config ARCH_ENABLE_MEMORY_HOTREMOVE
 
 config KEXEC
 	bool "kexec system call (EXPERIMENTAL)"
-	depends on BOOK3S && EXPERIMENTAL
+	depends on PPC_BOOK3S && EXPERIMENTAL
 	help
 	  kexec is a system call that implements the ability to shutdown your
 	  current kernel, and to start another kernel.  It is like a reboot