mpc85xx: p2020: Fix nand driver probe

mpc85xx: p2020: Fix nand driver probe

At this moment driver start fail with error:
[    3.771991] fsl,elbc-fcm-nand: probe of ffa00000.nand failed with error -22

elbc-fcm-nand driver use legacy method of ecc mode detection. It detect hw/sw
ecc mode when system configure it to "none". [1]

This patch adds 'nand-ecc-mode = "none"' propoerty to use generic driver
ecc mode detection.

[1] https://elixir.bootlin.com/linux/v5.10.18/source/drivers/mtd/nand/raw/fsl_elbc_nand.c#L730

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>