Workshop st: Add IPV4 feature to NUCLEO_F746ZG platform (#438)

* NUCLEO_F746ZG : enable Ethernet and LwIP for this target.
Net tests were passed :
mbed-os-features-feature_ipv4-tests-mbedmicro-net-nist_internet_time_service
| OK
mbed-os-features-feature_ipv4-tests-mbedmicro-net-tcp_client_hello_world
| OK
The 2 other are failing because I cannot test them on my ST computer.

Author: adustm

lwip-eth/arch/TARGET_STM/stm32f4_emac.c

@@ -1,4 +1,4 @@
-
+#if defined(TARGET_STM32F4)
 #include "stm32f4xx_hal.h"
 #include "lwip/opt.h"
 
@@ -552,7 +552,7 @@ void eth_arch_disable_interrupts(void)
 {
     NVIC_DisableIRQ(ETH_IRQn);
 }
-
+#endif // #if defined(TARGET_STM32F4)
 /**
  * @}
  */