microblaze: warn if of_iomap() failed

of_iomap() can return NULL. The function is void thus error propagation
is not possible but at least a WARN_ON() seems warranted here as a silent
failure might lead to a hard to understand system state.
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/pci/xilinx_pci.c

@@ -157,6 +157,7 @@ void __init xilinx_pci_init(void)
 
 	/* Set the max bus number to 255, and bus/subbus no's to 0 */
 	pci_reg = of_iomap(pci_node, 0);
+	WARN_ON(!pci_reg);
 	out_be32(pci_reg + XPLB_PCI_BUS, 0x000000ff);
 	iounmap(pci_reg);