avr32: remove support for AVR32 architecture

This patch drops support for AVR32 architecture from the Linux kernel.

The AVR32 architecture is not keeping up with the development of the
kernel, and since it shares so much of the drivers with Atmel ARM SoC,
it is starting to hinder these drivers to develop swiftly.

Also, all AVR32 AP7 SoC processors are end of lifed from Atmel (now
Microchip).

Finally, the GCC toolchain is stuck at version 4.2.x, and has not
received any patches since the last release from Atmel;
4.2.4-atmel.1.1.3.avr32linux.1. When building kernel v4.10, this
toolchain is no longer able to properly link the network stack.

Haavard and I have came to the conclusion that we feel keeping AVR32 on
life support offers more obstacles for Atmel ARMs, than it gives joy to
AVR32 users. I also suspect there are very few AVR32 users left today,
if anybody at all.

Signed-off-by: Hans-Christian Noren Egtvedt <egtvedt@samfundet.no>
Signed-off-by: Håvard Skinnemoen <hskinnemoen@gmail.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>