LPC2387: license cleanup

Rationale for this change (from #392):

> cpu/lpc2387/include/lpc23xx.h  doesn't give any license (only "All rights reserved.") - we should check if we can figure out if NXP specifies a license for that file somewhere.

[mbed](http://developer.mbed.org/users/mbed_official/code/mbed/file/82220227f4fa/LPC23xx.h) uses the same header file and states that the "the code in this repository is Apache licensed", so I guess it should be safe for us to add this license header, too. Does anyone disagree?