ralink: add missing nand layer hack

ralink: add missing nand layer hack

It turned out that mtk_nand driver uses some sophisticated accounting and a
general nand code must be patched.

This patch adds required read and erase calls to a general nand code. I
used a  code  for re6500 released by Linksys as a reference.

All required operations (erase, write and read) are usable. However I found
that jffs2 filesystem can be created only on top of a ubi volume. I tried
to create jffs2 directly on mtd device but pages with clean markers are
became uncorrectable.

Signed-off-by: Kirill Berezin <fyiwdt@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@47565 3c298f89-4303-0410-b956-a3cf2f4a3e73