ARM: OMAP2+: nand: header cleanup

For common arm zImage existing nand header file
in platform specific location was moved to generic
platform data location, but it contained more than
platform data, remove it. New local header has been
created for exposing functions.

Also move gpmc-nand platform data to platform header
meant for nand from gpmc header file

Signed-off-by: Afzal Mohammed <afzal@ti.com>