🧑‍💻 Allow DIGIPOT Rsx / Vrefmax override (#23885)

Marlin/src/feature/digipot/digipot_mcp4018.cpp

 
 // Settings for the I2C based DIGIPOT (MCP4018) based on WT150
 
-#define DIGIPOT_A4988_Rsx               0.250
-#define DIGIPOT_A4988_Vrefmax           1.666
-#define DIGIPOT_MCP4018_MAX_VALUE     127
+#ifndef DIGIPOT_A4988_Rsx
+  #define DIGIPOT_A4988_Rsx             0.250
+#endif
+#ifndef DIGIPOT_A4988_Vrefmax
+  #define DIGIPOT_A4988_Vrefmax         1.666
+#endif
+#define DIGIPOT_MCP4018_MAX_VALUE       127
 
 #define DIGIPOT_A4988_Itripmax(Vref)    ((Vref) / (8.0 * DIGIPOT_A4988_Rsx))