drivers/nrf24l01+ : added low-level functions for nrf24l01+ transceiver

This PR contains multiple low-level functions for the nrf24l01+ driver. Maybe some of them seem useless here but these are interesting  when it comes to bind the driver to the general network driver interface, which will be part of another PR soon. Furthermore there are some functions on which I'm not sure if they are necessary. Feel free to discuss :-) !
```diff
- * @ingroup     drivers_nrf24l01p
+ * @defgroup    drivers_nrf24l01p NRF24L01+ Driver Interface
+ * @ingroup     drivers
```