arm64: dts: freescale: imx8mm-verdin: add label to som adc node

Add a label to ti-ads1015 node to make it easier to reference it
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>