powerpc: Add device-tree model to the hardware description

Add the model of the machine we're on to the hardware description, which
is printed at boot and in case of an oops.

eg: Hardware name: IBM,8247-22L

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220930082709.55830-4-mpe@ellerman.id.au