mtd/ps3vram: Use _PAGE_NO_CACHE in memory ioremap

Use _PAGE_NO_CACHE for gpu memory ioremap.  Also,
add __iomem attribute to gpu memory pointer and
change use of memset() to memset_io().

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>