Allow overriding of dynamic keymap start address. (#18867)

quantum/dynamic_keymap.c

@@ -24,7 +24,9 @@
 #    include "via.h" // for VIA_EEPROM_CONFIG_END
 #    define DYNAMIC_KEYMAP_EEPROM_START (VIA_EEPROM_CONFIG_END)
 #else
-#    define DYNAMIC_KEYMAP_EEPROM_START (EECONFIG_SIZE)
+#    ifndef DYNAMIC_KEYMAP_EEPROM_START
+#        define DYNAMIC_KEYMAP_EEPROM_START (EECONFIG_SIZE)
+#    endif
 #endif
 
 #ifdef ENCODER_ENABLE