rename to sunxi_nand to avoid naming conflict

rename to sunxi_nand to avoid conflict with drivers/mtd/nand/nand.ko

discussion started here:  https://groups.google.com/d/topic/linux-sunxi/cYdLsiirve0/discussion

Essentially, with `sun7i_defconfig`, the sunxi_nand module never gets installed properly because it ends up having the same name as another kernel module.