cpu/lpc2387: rtc: only initialize RTC after cold boot or external reset
cpu/lpc2387: rtc: only initialize RTC after cold boot or external reset

On the MCB2388 plugging the power will result in both the POR and EXTR
bit being set.
Not sure if this is a property of the board, but it means RTC is also
reset after programming, so it behaves just like Backup RAM.

If we woke from Deep Sleep the POR bit will be cleared, so the RTC is not
reset.

authmillenon/RIOT,basilfx/RIOT,OTAkeys/RIOT,miri64/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,basilfx/RIOT,ant9000/RIOT,kYc0o/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,authmillenon/RIOT,OTAkeys/RIOT,jasonatran/RIOT,jasonatran/RIOT,jasonatran/RIOT,smlng/RIOT,kaspar030/RIOT,basilfx/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,kaspar030/RIOT,miri64/RIOT,yogo1212/RIOT,kYc0o/RIOT,ant9000/RIOT,basilfx/RIOT,authmillenon/RIOT,miri64/RIOT,basilfx/RIOT,kaspar030/RIOT,smlng/RIOT,yogo1212/RIOT,yogo1212/RIOT,OTAkeys/RIOT,smlng/RIOT,authmillenon/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,smlng/RIOT,ant9000/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kYc0o/RIOT,yogo1212/RIOT,ant9000/RIOT,OlegHahm/RIOT,smlng/RIOT,yogo1212/RIOT,OlegHahm/RIOT,kYc0o/RIOT