ARM: LPC32xx: Set system serial based on cpu unique id

LPC32xx SoC has a 128 bits unique id that can be used as a system
serial number, if none has been provided by atags or dt.

Signed-off-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Signed-off-by: Roland Stigge <stigge@antcom.de>