board: stm32mp1: correct the property name for eth

Use the correct name for STMicroelectronics phys config properties,
replace '_' by '-':
  "st,eth_clk_sel" => "st,eth-clk-sel"
  "st,eth-ref-clk-sel" => st,eth-clk-sel"

These property name are aligned with the upstreamed Linux kernel binding:
 linux/Documentation/devicetree/bindings/net/stm32-dwmac.yaml

See Linux kernel commit "dt-bindings: net: stmmac: add phys config
properties" merged in v5.1-rc1.

This patch allow to reuse the kernel device tree directly in U-Boot.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>