Documentation: devicetree: deprecate "soft_bch" nand-ecc-mode value

Now that we support nand-ecc-algo property it should be used together
with "soft" to specify software BCH ECC.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>