Fix RGB matrix for ATmegaxxU2 (#10723)

common_features.mk

@@ -184,6 +184,10 @@ ifeq ($(filter $(RGB_MATRIX_ENABLE),$(VALID_MATRIX_TYPES)),)
     $(error RGB_MATRIX_ENABLE="$(RGB_MATRIX_ENABLE)" is not a valid matrix type)
 endif
     OPT_DEFS += -DRGB_MATRIX_ENABLE
+ifneq (,$(filter $(MCU), atmega16u2 atmega32u2))
+    # ATmegaxxU2 does not have hardware MUL instruction - lib8tion must be told to use software multiplication routines
+    OPT_DEFS += -DLIB8_ATTINY
+endif
     SRC += $(QUANTUM_DIR)/color.c
     SRC += $(QUANTUM_DIR)/rgb_matrix.c
     SRC += $(QUANTUM_DIR)/rgb_matrix_drivers.c