[MTD] [NAND] atmel_nand: make available on AVR32

Make the atmel_nand driver selectable on AVR32, and update the Kconfig
help text to reflect this.

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