powerpc: Fix powerpc-specific watchdog build configuration

The powerpc kernel/watchdog.o should be built when HARDLOCKUP_DETECTOR
and HAVE_HARDLOCKUP_DETECTOR_ARCH are both selected. If only the former
is selected, then the generic perf watchdog has been selected.

To simplify this check, introduce a new Kconfig symbol PPC_WATCHDOG that
depends on both. This Kconfig option means the powerpc specific
watchdog is enabled.

Without this patch, Book3E will attempt to build the powerpc watchdog.

Fixes: 2104180a53 ("powerpc/64s: implement arch-specific hardlockup watchdog")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 36f858c..2a5060a 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -85,6 +85,17 @@
 	depends on SMP && (DEBUGGER || KEXEC_CORE || HARDLOCKUP_DETECTOR)
 	default y
 
+config PPC_WATCHDOG
+	bool
+	depends on HARDLOCKUP_DETECTOR
+	depends on HAVE_HARDLOCKUP_DETECTOR_ARCH
+	default y
+	help
+	  This is a placeholder when the powerpc hardlockup detector
+	  watchdog is selected (arch/powerpc/kernel/watchdog.c). It is
+	  seleted via the generic lockup detector menu which is why we
+	  have no standalone config option for it here.
+
 config STACKTRACE_SUPPORT
 	bool
 	default y