avr32: Retire AVR32 for good

AVR32 is gone. It's already more than two years for no support in Buildroot,
even longer there is no support in GCC (last version is heavily patched 4.2.4).

Linux kernel v4.12 got rid of it (and v4.11 didn't build successfully).

There is no good point to keep this support in U-Boot either.

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>