🔧 Update 644p/1284p Serial 1 sanity check (#24575)

Marlin/src/HAL/AVR/inc/SanityCheck.h

@@ -35,12 +35,20 @@
   || X_STEP_PIN == N || Y_STEP_PIN == N || Z_STEP_PIN == N \
   || X_DIR_PIN  == N || Y_DIR_PIN  == N || Z_DIR_PIN  == N \
   || X_ENA_PIN  == N || Y_ENA_PIN  == N || Z_ENA_PIN  == N \
+  || BTN_EN1    == N || BTN_EN2    == N \
 )
-#if CONF_SERIAL_IS(0) // D0-D1. No known conflicts.
+#if CONF_SERIAL_IS(0)
+  // D0-D1. No known conflicts.
 #endif
+#if NOT_TARGET(__AVR_ATmega644P__, __AVR_ATmega1284P__)
   #if CONF_SERIAL_IS(1) && (CHECK_SERIAL_PIN(18) || CHECK_SERIAL_PIN(19))
     #error "Serial Port 1 pin D18 and/or D19 conflicts with another pin on the board."
   #endif
+#else
+  #if CONF_SERIAL_IS(1) && (CHECK_SERIAL_PIN(10) || CHECK_SERIAL_PIN(11))
+    #error "Serial Port 1 pin D10 and/or D11 conflicts with another pin on the board."
+  #endif
+#endif
 #if CONF_SERIAL_IS(2) && (CHECK_SERIAL_PIN(16) || CHECK_SERIAL_PIN(17))
   #error "Serial Port 2 pin D16 and/or D17 conflicts with another pin on the board."
 #endif