staging: wfx: import HIF API headers

These files are shared with firmware sources. Only a subset of these
definitions are used by driver but, for now, it is easier to import all.

API defines 3 kinds of messages:
   - Requests (req) are sent from host to chip
   - Confirmations (cnf) are sent by chip and are always in reply to a
     request
   - Indications (ind) are spontaneous message from chip to host

One request normally generate one confirmation. There are a few
exceptions to this rule:
   - "shutdown" request is not acknowledged
   - multiple tx request can be acknowledged a unique "multi-tx"
     confirmation

In add, API defines MIB. They are sub-structures for write_mib and
read_mib API.

Note that all numbers in API have to be little endian when sent/received
from/to chip (I didn't declared them with __le32 because driver also use
them internally).
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20190919142527.31797-7-Jerome.Pouiller@silabs.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>