powerpc/64s: Make reliable stacktrace dependency clearer

Make the HAVE_RELIABLE_STACKTRACE Kconfig option depend on
PPC_BOOK3S_64 for documentation purposes. Before this patch, it
depended on PPC64 && CPU_LITTLE_ENDIAN and because CPU_LITTLE_ENDIAN
implies PPC_BOOK3S_64, there's no functional change here.
Signed-off-by: Nicolai Stange <nstange@suse.de>
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
[mpe: Split out of larger patch]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/Kconfig

@@ -220,7 +220,7 @@ config PPC
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_RCU_TABLE_FREE		if SMP
 	select HAVE_REGS_AND_STACK_ACCESS_API
-	select HAVE_RELIABLE_STACKTRACE		if PPC64 && CPU_LITTLE_ENDIAN
+	select HAVE_RELIABLE_STACKTRACE		if PPC_BOOK3S_64 && CPU_LITTLE_ENDIAN
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_VIRT_CPU_ACCOUNTING
 	select HAVE_IRQ_TIME_ACCOUNTING