arm: mvebu: turris_omnia: fixup MTD partitions in Linux' DTB

Fixup SPI NOR partition nodes in Linux' device tree prior booting Linux.

Linux' devicetree does not contain "u-boot-env" partition and we do not
want to add it there because the address is different between stock
U-Boot and current upstream U-Boot.

Instead we add code that recreates partition nodes from scratch
according to how U-Boot sees them (which is defined in U-Boot's device
tree).

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>