ARM: 8354/1: Documentation: devicetree: root node serial-number property documentation

Open firmware is already using the serial-number property for passing the
device's serial number from the bootloader to the kernel. In addition, lshw
already has support for scanning this property.

The serial number is a string that somewhat represents the device's serial
number. It might come from some form of storage (e.g. an eeprom) and be
programmed at factory-time by the manufacturer or come from identification
bits available in e.g. the SoC (note that the soc_id property in the SoC bus
should hold a full account of those bits).

The serial number is taken as-is from the bootloader, so it is up to the
bootloader to define where the serial number comes from and what length it
should be. Some use cases for the serial number require it to have a maximum
length (e.g. for USB serial number) and some other cases imply more restrictions
on what the serial number should look like (e.g. in Android, the ro.serialno
property is usually a 16-bytes (plus one null byte) representation of a 64 bit
number).

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>