cpu/lpc2387: Fixed doc and attribute of arm_reset

- arm_reset was completely undocumented, even though technical details buried
  deeply in the data sheet of the LPC2387 are involved in the code
- The attribute "naked" is misplaced, it should only be used when no C code
  is present. However, the function consists of C code only
- The attribute "noreturn" has to be used in the declaration [1] of a function,
  not in the implementation. Otherwise the caller is not informed and code using
  the function will not be optimized.

[1]: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html