mtd: onenand: samsung: remove incorrect __iomem annotation

'page_buf' and 'oob_buf' are allocated with 'devm_kzalloc()' and should not
have __iomem decoration.

Remove these decorations and some useless casting.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>