Add Z_MIN_PROBE_PIN to MKS SBASE pins

Marlin/src/pins/pins_MKS_SBASE.h

 #define Z_MIN_PIN          P1_28   // The original Mks Sbase DIO19 has a 10k pullup to 3.3V or 5V, 1K series, so when using a Zprobe we must use DIO41 (J8 P1.22)
 #define Z_MAX_PIN          P1_29   // 10k pullup to 3.3V, 1K series
 
+#ifndef Z_MIN_PROBE_PIN
+  #define Z_MIN_PROBE_PIN  P4_28   // Connector J8
+#endif
+
 //
 // Steppers
 //
 #define PIN_P1_23          P1_23   // PWM Capable
 #define PIN_P2_12          P2_12   // Interrupt Capable
 #define PIN_P2_11          P2_11   // Interrupt Capable
-#define PIN_P4_28          P4_28
 
 //
 // Prusa i3 MK2 Multi Material Multiplexer Support