arm64: dts: freescale: imx8mp-verdin: add labels to som nodes

Add labels to ti-ads1015 and fec ethernet mdio node to make it easier to
reference them from other nodes.

Signed-off-by: João Paulo Gonçalves <joao.goncalves@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>