[MTD] [NAND] rename at91_nand -> atmel_nand: file names and Kconfig

The AT91 NAND driver needs just a few tiny modifications to work on
AVR32 as well. Rename it atmel_nand to reflect this.

Also move the ECC register definitions into drivers/mtd/nand since they
are only useful to the atmel_nand driver, and get rid of the useless
filename at the top of each file.

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