[MTD] [NAND] rename at91_nand -> atmel_nand: internal symbols

This is basically s/at91_nand/atmel_nand/g with some manual inspection.

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