arm: mvebu: dts: turris_mox: add nodes for SPI NOR partitions

Add nodes for SPI NOR partitions to the device tree of Turris MOX, as
are in Linux' device tree.

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