Save lots of PROGMEM, ~20b SRAM with DIGIPOT_I2C

Marlin/SanityCheck.h

@@ -1165,6 +1165,16 @@ static_assert(1 >= 0
   #error "Enable STEALTHCHOP to use HYBRID_THRESHOLD."
 #endif
 
+/**
+ * Digipot requirement
+ */
+#if ENABLED(DIGIPOT_MCP4018)
+  #if !defined(DIGIPOTS_I2C_SDA_X) || !defined(DIGIPOTS_I2C_SDA_Y) || !defined(DIGIPOTS_I2C_SDA_Z) \
+    || !defined(DIGIPOTS_I2C_SDA_E0) || !defined(DIGIPOTS_I2C_SDA_E1)
+      #error "DIGIPOT_MCP4018 requires DIGIPOTS_I2C_SDA_* pins to be defined."
+  #endif
+#endif
+
 /**
  * Require 4 or more elements in per-axis initializers
  */