mtd: atmel_nand: enable Nand Flash Controller (NFC) read data via sram

NFC has embedded sram which can use to transfer data. This patch enable reading
nand flash via NFC SRAM. It will minimize the CPU overhead.

This driver has been tested on SAMA5D3X-EK with JFFS2, YAFFS2, UBIFS and
mtd-utils.

Here puts the part of mtd_speedtest (read test) result as following:
Compare with non-NFC mtd_speedtest result, reading will reduce %45 cpu load
with increase %80 speed.

- commands use to test:
  # insmod /mnt/mtd_speedtest.ko dev=2 &
  # top -n 30 -d 1 | grep speedtest

- test result:
=================================================
mtd_speedtest: MTD device: 2
mtd_speedtest: MTD device size 41943040, eraseblock size 131072, page size 2048, count of eraseblocks 320, pages per eraseblock 64, OOB size 64
mtd_speedtest: testing eraseblock read speed
  509   495 root     D     1164   0%  28% insmod /mnt/mtd_speedtest.ko dev=2
  509   495 root     D     1164   0%  25% insmod /mnt/mtd_speedtest.ko dev=2
  509   495 root     D     1164   0%  26% insmod /mnt/mtd_speedtest.ko dev=2
mtd_speedtest: eraseblock read speed is 9403 KiB/s
mtd_speedtest: testing page read speed
  509   495 root     R     1164   0%  31% insmod /mnt/mtd_speedtest.ko dev=2
  509   495 root     D     1164   0%  57% insmod /mnt/mtd_speedtest.ko dev=2
  509   495 root     D     1164   0%  53% insmod /mnt/mtd_speedtest.ko dev=2
  509   495 root     D     1164   0%  71% insmod /mnt/mtd_speedtest.ko dev=2
mtd_speedtest: page read speed is 9258 KiB/s

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>