powerpc/powernv: Add opal details to the hardware description

Add OPAL version details to the hardware description, which is printed
at boot and in case of an oops.

eg: Hardware name: ... opal:v6.2

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220930082709.55830-5-mpe@ellerman.id.au