powerpc/topology: Fix spurr attribute permission

We are registering the attribute with permission 0600 but it
doesn't have a store callback, which causes WARN_ON's during
boot. Fix the permission.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/sysfs.c

@@ -180,7 +180,7 @@ SYSFS_PMCSETUP(dscr, SPRN_DSCR);
 SYSFS_PMCSETUP(pir, SPRN_PIR);
 
 static DEVICE_ATTR(mmcra, 0600, show_mmcra, store_mmcra);
-static DEVICE_ATTR(spurr, 0600, show_spurr, NULL);
+static DEVICE_ATTR(spurr, 0400, show_spurr, NULL);
 static DEVICE_ATTR(dscr, 0600, show_dscr, store_dscr);
 static DEVICE_ATTR(purr, 0600, show_purr, store_purr);
 static DEVICE_ATTR(pir, 0400, show_pir, NULL);