Fix MKS Robin Nano V3 I2C pins (#21174)

buildroot/share/PlatformIO/variants/MARLIN_F4x7Vx/variant.h

@@ -143,8 +143,12 @@ extern "C" {
 #define PIN_SPI_SCK             PA5
 
 // I2C definitions
-#define PIN_WIRE_SDA            PB9
-#define PIN_WIRE_SCL            PB8
+#ifndef PIN_WIRE_SDA
+  #define PIN_WIRE_SDA          PB9
+#endif
+#ifndef PIN_WIRE_SCL
+  #define PIN_WIRE_SCL          PB8
+#endif
 
 // Timer Definitions
 // Use TIM6/TIM7 when possible as servo and tone don't need GPIO output pin