drivers: ethernet: stm32_hal: Make MAC address setting method a K choice

In order to ease deprecation of these methods (in favor of dt based
configuration), introduce a choice for MAC address configuration method.

Default to random method (no change).

User wanting to still use ETH_STM32_HAL_MACX symbols should define
CONFIG_ETH_STM32_HAL_USER_STATIC_MAC=y

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>