SAME5x support - #585

Summary

Adds TinyUSB support for SAME5x SAM E-series devices, while preserving the existing SAMD21 and SAMD51 paths.

This change:

• maps SAME53/SAME54 Arduino core symbols to OPT_MCU_SAME5X
• adds a private SAME5x USB support header for the SAMD TinyUSB driver
• wires SAME5x USB interrupt handlers to TinyUSB
• enables SAME5x device and host IRQ setup in the SAMD portable driver
• uses TinyUSB TU_CHECK_MCU(...) checks consistently after CFG_TUSB_MCU is established
• adjusts MAX3421 SAMD SPI clock limits so SAMD21 stays at 12 MHz while SAMD51/SAME5x can request 25 MHz

Limitations

This follows the SAMD51 USB peripheral shape with SAME5x register naming support. It has been hardware tested on SAME54, but not across every SAME5x package/variant.

Testing

• Ran pre-commit run --all-files
• Ran the TinyUSB Arduino CI board matrix locally in an isolated Arduino CLI environment
• Ran SimIO SAME54 control hub hardware smoke tests:
  • TinyUSB device controller initializes
  • USB task runs