- Move oob layout to driver
- make hwecc support nonoptional but selectable by the board.
- let the boards decide which buswidth / ecc type to use. They know
  better than the user which gets confused by too many kconfig options.
- Let the boards explicitly register the nand device.

I tried to seperate this further, but failed because of too many dependencies
in the code.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>