ARM: atmel: sama5d4: move usb device init code to common place

Move usb device initialize code to common place, then it can be
shared by other board which use sama5d4 SoC.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
ARM: atmel: sama5d4 xplained: add option to enable usb ethernet

Make it configurable for USB Ethernet.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
ARM: atmel: sama5d4 xplained: enable usb ethernet gadget

Signed-off-by: Bo Shen <voice.shen@atmel.com>