powerpc/nvram: Move the log partition stuff to pseries

The nvram log partition stuff currently in nvram_64.c is really
pseries specific. It isn't actually used on anything else (despite
the fact that we ran the code to setup the partition on anything
except powermac) and the log format is specific to pseries RTAS
implementation. So move it where it belongs

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>