[media] media: s5p-mfc: fix sparse warnings

Commits a0f10c13 and
6c9fe765 ("media: s5p-mfc: fix broken
pointer cast on 64bit arch") fixed issue with lossy cast on 64-bit
architectures. However it also removed __iomem attribute from that cast.
This leads to sparse warnings. This patch fixes those warnings by adding
__iomem cast in case of v6+ code version and replacing readl/writel by
simple u32 load/store operations in case of v5 code (which is called on
system memory allocated by dma_alloc_coherent() instead of io registers).

Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>