powerpc: iommu: fix build when neither PCI or IBMVIO is set

When neither CONFIG_PCI nor CONFIG_IBMVIO is set/enabled, iommu.c has a
build error. The fault injection code is not useful in that kernel config,
so make the FAIL_IOMMU option depend on PCI || IBMVIO.

Prevents this build error (warning escalated to error):
../arch/powerpc/kernel/iommu.c:178:30: error: 'fail_iommu_bus_notifier' defined but not used [-Werror=unused-variable]
  178 | static struct notifier_block fail_iommu_bus_notifier = {

Fixes: d6b9a81b ("powerpc: IOMMU fault injection")
Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210404192623.10697-1-rdunlap@infradead.org

arch/powerpc/Kconfig.debug

@@ -353,6 +353,7 @@ config PPC_EARLY_DEBUG_CPM_ADDR
 config FAIL_IOMMU
 	bool "Fault-injection capability for IOMMU"
 	depends on FAULT_INJECTION
+	depends on PCI || IBMVIO
 	help
 	  Provide fault-injection capability for IOMMU. Each device can
 	  be selectively enabled via the fail_iommu property.