MAINTAINERS: Add decsny as ETH/MDIO collaborator

Add myself (decsny) as ethernet/mdio collaborator.

I have taken an interest in ethernet and am currently
maintaining and writing some ethernet/mdio drivers for NXP,
and would like to use the collaborator role to monitor the
zephyr activity of the ethernet/mdio subsystems.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>