Orion5x: Fix warning: struct pci_dev declared inside paramter list

Orion5x board files which don't have PCI give warnings:

arch/arm/mach-orion5x/common.h:54:38: warning: 'struct pci_dev' declared
inside parameter list.

Add a forward declaration in the header file, which is the pattern
used for other PCI structures.
Signed-off-by: NAndrew Lunn <andrew@lunn.ch>

arch/arm/mach-orion5x/common.h

@@ -45,6 +45,7 @@ void orion5x_restart(char, const char *);
  */
 struct pci_bus;
 struct pci_sys_data;
+struct pci_dev;
 
 void orion5x_pcie_id(u32 *dev, u32 *rev);
 void orion5x_pci_disable(void);