configs: j7200_evm_a72: Add CONFIG_PREBOOT to configure ethernet PHY

Add CONFIG_PREBOOT to provide an automatic and easier way
to configure ethernet PHY before loading the firmware.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210721155849.20994-19-kishon@ti.com