net: dwc_eth_qos: Rename eqos_stm32_config to eqos_stm32mp15_config

net: dwc_eth_qos: Rename eqos_stm32_config to eqos_stm32mp15_config

The current glue code is specific to STM32MP15xx, the upcoming STM32MP13xx
will introduce another entry specific to the STM32MP13xx. Rename the current
entry to eqos_stm32mp15_config in preparation for STM32MP13xx addition. No
functional change.

Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Christophe ROULLIER <christophe.roullier@foss.st.com>