i2c: mxc_i2c: Document how non-DM functions work

It is not very clear how these work in relation to the exact I2C xfers
they produce.  In paticular, the address length is somewhat overloaded
in the read method.  Clearly document the existing behavior.  Maybe this
will help the next person who needs to work on this driver and not break
non-DM boards.

Cc: Nandor Han <nandor.han@ge.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Breno Matheus Lima <brenomatheus@gmail.com>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>