NAND: change read delay from absolut to RnB based

BugLink: http://bugs.launchpad.net/bugs/611634

Signed-off-by: Tawfic Bayyouk <tawficb@gmail.com>
Signed-off-by: Eric Miao <eric.miao@canonical.com>
Signed-off-by: Brad Figg <brad.figg@canonical.com>