powerpc/pseries: Add firmware details to the hardware description

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

Use /hypervisor if we find it, though currently it only exists if we're
running under qemu.

Look for "ibm,powervm-partition" which is specified in PAPR+ v2.11 and
tells us we're running under PowerVM.

Failing that look for "ibm,fw-net-version" which is seen on PowerVM
going back to at least Power6.

eg: Hardware name: ... of:IBM,FW860.42 (SV860_138) hv:phyp

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220930082709.55830-6-mpe@ellerman.id.au