avr32: Add support for ATSTK1006

The ATSTK1006 is basically an upgraded version of ATSTK1002 with
128 MiB SDRAM and 256 MiB NAND flash on board.

Otherwise, the board is very similar to the ATSTK1002, so it uses the
same board support file.

Signed-off-by: Håvard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>