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: Andrew Lunn <andrew@lunn.ch>