nand: eliminate some sunxi-specific code dealing with device nodes.

Since generic partition code is used no need to have partition code in
sunxi driver.

Note that the device minor number possibly changes:

brw-rw---T 1 root disk 93,  0 Feb 28 14:24 /dev/nand
brw-rw---T 1 root disk 93,  1 Feb 28 14:24 /dev/nand1
brw-rw---T 1 root disk 93,  2 Feb 28 14:24 /dev/nand2
brw-rw---T 1 root disk 93,  8 Feb 28 14:24 /dev/nanda
brw-rw---T 1 root disk 93, 16 Feb 28 14:24 /dev/nandb