mtd: nand: omap2: Fix OMAP_BCH option dependency

This option does not need to depend in MTD_NAND, for it's enclosed
under it. Also, it's wrong to make it depend in ARCH_OMAP3 only
since the controller is used in a wider range of SoCs.

Instead, just leave the dependency on the OMAP2 driver option.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Acked-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>