Documentation: dt: mtd: drop "soft_bch" from example

Commit 32698aaf (Documentation: devicetree: deprecate "soft_bch"
nand-ecc-mode value, 2016-04-22) deprecated "soft_bch". Update the example to
match.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>