hw/misc/max111x: Create header file for documentation, TYPE_ macros

hw/misc/max111x: Create header file for documentation, TYPE_ macros

Create a header file for the hw/misc/max111x device, in the
usual modern style for QOM devices:
 * definition of the TYPE_ constants and macros
 * definition of the device's state struct so that it can
   be embedded in other structs if desired
 * documentation of the interface

This allows us to use TYPE_MAX_1111 in the spitz.c code rather
than the string "max1111".

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-id: 20200628142429[email protected]