ARM: dts: armada-385-turris-omnia: add `u-boot-env` NOR partition

Specify a separate partition `u-boot-env` for U-Boot's env settings for
the Turris Omnia board.

Do this only in U-Boot's specific DTS. We do not want to do this in
Linux' official DTS, because Omnia's stock U-Boot stores env at a
different address, and there are still boards with stock U-Boot.

In a subsequent patch will add board code that fixes Linux's DTB before
booting Linux.

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