sparc64: simplify kernel config, fix virtio disk boot

target/sparc64/kernel/qemu-sparc64

@@ -2,9 +2,5 @@ CONFIG_SPARC=y
 CONFIG_SPARC64=y
 CONFIG_SUN_OPENPROMFS=y
 CONFIG_SPARC64_PCI=y
-CONFIG_OF=y
-CONFIG_OF_PROMTREE=y
-CONFIG_OF_NET=y
-CONFIG_OF_PCI=y
-CONFIG_OF_IOMMU=y
 CONFIG_IOMMU_SUPPORT=y
+CONFIG_CMDLINE_BOOL=y