northbridge/amd/agesa/familyXY: Make NULL device op explicit

Use 'DEVICE_NOOP' macro introduced in:

 commit 530355d include/device/device.h: Provide DEVICE_NOOP macro shim

to provide formalism. Make the null device ops here explicit and
in-line with formalism elsewhere.

Change-Id: I2400b29a5108a6bae21959177e53321810ca1407
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/8035
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marc.jones@se-eng.com>
diff --git a/src/northbridge/amd/agesa/family15tn/northbridge.c b/src/northbridge/amd/agesa/family15tn/northbridge.c
index d878b4f..a18da68 100644
--- a/src/northbridge/amd/agesa/family15tn/northbridge.c
+++ b/src/northbridge/amd/agesa/family15tn/northbridge.c
@@ -955,7 +955,7 @@
 	.read_resources	  = domain_read_resources,
 	.set_resources	  = domain_set_resources,
 	.enable_resources = domain_enable_resources,
-	.init		  = NULL,
+	.init		  = DEVICE_NOOP,
 	.scan_bus	  = pci_domain_scan_bus,
 	.ops_pci_bus	  = pci_bus_default_ops,
 };