The SPI NOR flash node name in main device tree for Turris Omnia is
called `spi-nor@0`.

Rename node spi-flash@0 in Turris Omnia's -u-boot.dtsi file to spi-nor@0
so that U-Boot does not try to probe the same SPI NOR device multiple
times.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Stefan Roese <sr@denx.de>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Pali Rohár <pali@kernel.org>