MIPS: jz4740: Move jz4740_nand.h header to include/linux/platform_data/jz4740

This way we will be able to compile the jz4740_nand driver when
COMPILE_TEST=y.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>