drivers: ieee802154: document endianness

The IEEE 802.15.4 radio driver encodes attributes in:

* little endian for everything that is close to the protocol as
  IEEE 802.15.4 frames are little endian encoded.

* mixed big and little endian in its configuration where extended
  addresses are being represented. These inconsistencies are unfortunate
  but cannot be easily fixed in a backwards compatible way so will be
  left untouched in this change.

Endianness was almost nowhere documented which explains these
inconsistencies and led to several bugs where assignments of different
byte order are not converted (or sometimes converted, sometimes not).

This change documents intended endianness within the realm of the
IEEE 802.15.4 radio driver code. Conversion bugs are fixed in a separate
commit.

Signed-off-by: Florian Grandel <jerico.dev@gmail.com>