add DEBUG_MATRIX_SCAN_RATE_ENABLE to common_features.mk (qmk#10824)

Add a Make variable to easily enable DEBUG_MATRIX_SCAN_RATE on the command line.

eg.
```
make DEBUG_MATRIX_SCAN_RATE_ENABLE=yes  KEYBOARD:KEYMAP
```

Author: mtei

common_features.mk

@@ -20,6 +20,11 @@ QUANTUM_SRC += \
     $(QUANTUM_DIR)/keymap_common.c \
     $(QUANTUM_DIR)/keycode_config.c
 
+ifeq ($(strip $(DEBUG_MATRIX_SCAN_RATE_ENABLE)), yes)
+    OPT_DEFS += -DDEBUG_MATRIX_SCAN_RATE
+    CONSOLE_ENABLE = yes
+endif
+
 ifeq ($(strip $(API_SYSEX_ENABLE)), yes)
     OPT_DEFS += -DAPI_SYSEX_ENABLE
     OPT_DEFS += -DAPI_ENABLE

tmk_core/common/keyboard.c

@@ -299,6 +299,10 @@ void keyboard_init(void) {
     dip_switch_init();
 #endif
 
+#if defined(DEBUG_MATRIX_SCAN_RATE) && defined(CONSOLE_ENABLE)
+    debug_enable = true;
+#endif
+
     keyboard_post_init_kb(); /* Always keep this last */
 }