powerpc/powernv: rename remaining rng powernv_ functions to pnv_

The preferred nomenclature is pnv_, not powernv_, but rng.c used
powernv_ for some reason, which isn't consistent with the rest. A recent
commit added a few pnv_ functions to rng.c, making the file a bit of a
mishmash. This commit just replaces the rest of them.

Fixes: f3eac426 ("powerpc/powernv: wire up rng during setup_arch")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
[mpe: Reorder after bug fix commits]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220727143219.2684192-3-mpe@ellerman.id.au