arm64: dts: imx8mm-phycore-som: Add no-eth phy overlay

There are SoM variants with no eth phy populated. Add overlay to be
able to support this.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Signed-off-by: Andrej Picej <andrej.picej@norik.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>