ARM: dts: stm32: move stmmac axi config in ethernet node on stm32mp15

It fixes the following warning seen running "make dtbs_check W=1"

Warning (simple_bus_reg): /soc/stmmac-axi-config: missing or empty
reg/ranges property

Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>