ar71xx: bring back Atheros AP83 support

ar71xx: bring back Atheros AP83 support

I still have a (working!) AP83-040 board, so let's bring back
support for it. Changes since it was dropped:

 - WLAN LED isn't working, "fixme"
 - hardcoded flash layout was added back into the mach file
 - sysupgrade is "fixme"
 - the flash mechanism and the custom SPI driver was not
   modernized, but they compile and work.

Other than these notes, the board is working as expected.

Signed-off-by: Zoltan HERPAI <[email protected]>