mtd: nand: davinci: add comment on NAND subpage write status on keystone

Add a comment clarifying that NAND subpage write on keystone works,
but is not being enabled in the interest of backward compatibility.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>