powerpc: Correct dependency of KEXEC

commit 28794d34ecb6815a3fa0a4256027c9b081a17c5f ("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>
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index e0f0a4d..5b50e1a 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -316,7 +316,7 @@
 
 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