net: dwc_eth_qos: add Kconfig option to select supported configuration

Add configuration flag to select the supported dwc driver configuration:
- CONFIG_DWC_ETH_QOS_TEGRA186
- CONFIG_DWC_ETH_QOS_IMX
- CONFIG_DWC_ETH_QOS_STM32

See Linux driver ethernet/stmicro/stmmac and associated glue layers
for other configuration examples.

This patch removes the not-selected compatibles and lets the linker remove
the unused functions to reduce the size of the driver.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>