ppc/pnv: Update skiboot to v7.1

This includes a number of improvements and fixes. Importantly there
is a change for QEMU platforms to permit the ChipTOD to be initialised
if it is present in the device tree. This will facilitate ChipTOD
enablement in pnv.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>