cpu/board: added support for nrf51822

- added cpu Nordic `nrf51822`
- added board `pca10000` (from Nordic Dev Kit)
- added board `pca10005` (also from Nordic Dev Kit)
insert the definition for the bords stdio here:
```
/**
 * @name Define the boards stdio
 * @{
 */
#define STDIO               UART_0
#define STDIO_BAUDRATE      (115200U)
/** @} */
```