powerpc/64s: Make reliable stacktrace dependency clearer

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>