Fix rename of at91_nand -> atmel_nand

Structs called at91_nand_data where renamed to atmel_nand_data
and configs called *MTD_NAND_AT91* where renamed to
*MTD_NAND_ATMEL*. This was unfortunately not done consistently,
causing NAND chips not being initialised on several ARM boards.

I am aware that the author of the original change did not rename
MTD_NAND_AT91_BUSWIDTH to MTD_NAND_ATMEL_BUSWIDTH, for example.
All *MTD_NAND_AT91* where renamed to *MTD_NAND_ATMEL* in order
to keep naming consistency.

This patch was only tested on a MACH_SAM9_L9260, as this is the
only ARM board I have to my disposal.

Before this patch:

$ git-ls-files |xargs grep atmel_nand |wc -l
105
$ git-ls-files |xargs grep at91_nand |wc -l
4
$ git-ls-files |xargs grep MTD_NAND_ATMEL |wc -l
8
$ git-ls-files |xargs grep MTD_NAND_AT91 |wc -l
47

After this patch:

$ git-ls-files |xargs grep atmel_nand |wc -l
109
$ git-ls-files |xargs grep at91_nand |wc -l
0
$ git-ls-files |xargs grep MTD_NAND_ATMEL |wc -l
55
$ git-ls-files |xargs grep MTD_NAND_AT91 |wc -l
0

Signed-off-by: Pieter du Preez <pdupreez@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>