mtd: nand: sunxi: introduce sunxi_nfc_hw_ecc_read/write_chunk()

The logic behind normal and syndrome ECC handling is pretty much the same,
the only difference is the ECC bytes placement.
Create two functions to read/write ECC chunks. Those functions will later
be used by the sunxi_nfc_hw_ecc_read/write_page() and
sunxi_nfc_hw_syndrome_ecc_read/write_page() functions.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>