arm: mvebu: Clean up Thecus N2350 board DTS

- Update the Thecus N2350 DTS to conform with latest device-tree binding
and styles.
- Correct typo in mdio node.

Signed-off-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Pali Rohár <pali@kernel.org>