mtd: nand: xway: add some more documentation

This adds some register documentation which should make it easier to
understand how this controller works. In addition it makes now use of
BIT() macro and adds some more defines.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>