Fix onboard SD card support for Teensy 3.6 & 4.1 (MarlinFirmware#19593)

Author: bilsef

Marlin/src/sd/Sd2Card.h

@@ -84,6 +84,11 @@ uint8_t const SD_CARD_TYPE_SD1  = 1,                    // Standard capacity V1
   #define SOFTWARE_SPI
 #endif
 
+#if IS_TEENSY_35_36 || IS_TEENSY_40_41
+  #include "NXP_SDHC.h"
+  #define BUILTIN_SDCARD 254
+#endif
+
 /**
  * \class Sd2Card
  * \brief Raw access to SD and SDHC flash memory cards.