nand: pmecc: disable the 0xff page check for SAMA5D4 chip

As SAMA5D4 chip PMECC can correct 0xff page bitflips. So we can avoid
the workaround for 0xff page check when correct bit flips.

In this patch, we adds a PMECC IP version check. If PMECC IP version is
less than PMECC_SAMA5D4_VERSION, then we need use workaround to check
0xff page. Otherwise skip the check.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
README: refine the readme file

1. fix typo.
2. avoid longer than 80 characters in a line.
3. update the cross-compiled toolchains.
4. reorgnize the paragraph.
5. add the release command.
4. add contributing part.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>