Section B.6.2.1 Root Node Properties of PAPR specification defines
a set of properties which shall be present in the device tree root,
one of these properties is "system-id" which "should be unique across
all systems and all manufacturers". Since UUID is meant to be unique,
it makes sense to use it as "system-id".

This adds "system-id" property to the device tree root when not empty.
Signed-off-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>