mtd: onenand: omap2: obtain memory from resource

gpmc initialization for onenand done by platform code now provides
onenand address space as memory resource. Hence remove usage of
gpmc_cs_request in onenand driver and obtain memory details from
resource structure.

Signed-off-by: Afzal Mohammed <afzal@ti.com>
Acked-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>