omap: cleanup NAND platform data

omap_nand_platform_data fields 'options', 'gpio_irq', 'nand_setup' and
'dma_channel' are never referenced by the NAND driver, yet various
board files are initializing those fields. This is both incorrect and
confusing, so remove them. This allows to get rid of a global
variable in gpmc-nand.c.

This also corrects an issue where some boards are trying to pass NAND
16bit flag through .options, but the driver is using .devsize instead
and ignoring .options.

Finally, .dev_ready is treated as a flag by the driver, so make it bool
instead of a function pointer.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>