How about this patch? (not tested at all)
---
Izumi Tsutsui
tsutsui@ceres.dti.ne.jp

Index: arch/prep/prep/ibm_7248.c
===================================================================
RCS file: /cvsroot/src/sys/arch/prep/prep/ibm_7248.c,v
retrieving revision 1.7
diff -u -r1.7 ibm_7248.c
--- arch/prep/prep/ibm_7248.c	15 Jul 2003 02:54:52 -0000	1.7
+++ arch/prep/prep/ibm_7248.c	13 Feb 2004 12:44:14 -0000
@@ -46,6 +46,11 @@
 
 static void pci_intr_fixup_ibm_7248(int, int, int *);
 
+static const char *obiodevs_ibm_7248[] = {
+	"wdc",
+	NULL
+};
+
 struct platform platform_ibm_7248 = {
 	"IBM PPS Model 7248 (E)",		/* model */
 	platform_generic_match,			/* match */
@@ -54,7 +59,7 @@
 	init_intr_ivr,				/* init_intr */
 	cpu_setup_ibm_generic,			/* cpu_setup */
 	reset_prep_generic,			/* reset */
-	obiodevs_nodev,				/* obiodevs */
+	obiodevs_ibm_7248,			/* obiodevs */
 };
 
 static void