powerpc/configs/64s: Enable PSTORE

Like pseries & powernv_defconfig, enable PSTORE.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230414132415.821564-27-mpe@ellerman.id.au

arch/powerpc/configs/ppc64_defconfig

@@ -341,6 +341,7 @@ CONFIG_SQUASHFS=m
 CONFIG_SQUASHFS_XATTR=y
 CONFIG_SQUASHFS_LZO=y
 CONFIG_SQUASHFS_XZ=y
+CONFIG_PSTORE=y
 CONFIG_NFS_FS=y
 CONFIG_NFS_V3_ACL=y
 CONFIG_NFS_V4=y