mtd: onenand: omap2: Configure driver from DT

Move away from platform data configuration and use pure DT approach.

Use generic probe function to deal with OneNAND node and remove now useless
gpmc_probe_onenand_child function. Import sync mode timing calculation
function from mach-omap2/gpmc-onenand.c

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>