device-tree: Document the rpi-serial64 device-tree node

Author: timg236

documentation/asciidoc/computers/configuration/device-tree.adoc

@@ -845,6 +845,8 @@ The firmware uses the special https://www.kernel.org/doc/html/latest/devicetree/
 
 `rpi-duid`:: _(string)_ Raspberry Pi 5 only. A string representation of the QR code on the PCB.
 
+`rpi-serial64`:: _(string)_ A string representation of the 64-bit serial number. On flagship models since Raspberry Pi 5 this is same as the normal serial number (`/proc/device-tree/serial-number`). On earlier models the default serial number is still 32-bit but with newer firmware a 64-bit serial number is now available and is visible through this node.
+
 ==== Common bootloader properties `/chosen/bootloader`
 
 Each property is stored as a 32-bit integer unless indicated otherwise.

documentation/asciidoc/computers/raspberry-pi/otp-bits.adoc

@@ -54,6 +54,7 @@ This is available via device-tree in `/proc/device-tree/chosen/rpi-boardrev-ext`
 * Raspberry Pi 400
  ** Bits 0-7: The default keyboard country code used by https://github.com/raspberrypi-ui/piwiz[piwiz]
 
+`35` :: High 32 bits of 64-bit serial number
 `36-43`:: xref:raspberry-pi.adoc#industrial-use-of-the-raspberry-pi[customer OTP values]
 `45`:: MPG2 decode key
 `46`:: WVC1 decode key