ARM: dts: at91: Fix USB endpoint nodes

Endpoint nodes have a reg property. Add their mandatory unit-address.

This solves:
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep0 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep1 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep2 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep3 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep4 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep5 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep6 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep7 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep8 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep9 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep10 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep11 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep12 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep13 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep14 has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): Node /ahb/gadget@00400000/ep15 has a reg or ranges property, but no unit name

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>