yaffs2: include rawnand.h instead of nand.h

The nand.h has been renamed to rawnand.h by commit d4092d76 ("mtd:
nand: Rename nand.h into rawnand.h"), adjust the yaffs2 code according
to this change.

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>