arm64: dts: imx8mp-phycore-som: Reduce drive strength for fec tx lines

Reduce drive strength on fec tx lines for signal quality improvements.
Measurements showed that TD0 and TD1 require X4 and the other lines
X2 for optimized settings.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>