mtd: nand: mxc: Add own write_page

Now that we have our own read_page function add a write_page function
for consistency aswell. This can be a lot easier than the generic
function since we do not have to iterate over subpages but can write
the whole page at once. Also add write_page_raw and write_oob for
proper raw and oob write support.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>