nand: pxa3xx: cosmetic: add comments to the timing layout structures

Add comments with timing parameter names and some details about
nand layout fileds.
Remove unneeded definition.

Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Reviewed-by: Igal Liberman <igall@marvell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Stefan Roese <sr@denx.de>