mtd: nand: rename the cellinfo to bits_per_cell

The @cellinfo fields contains unused information, such as write caching,
internal chip numbering, etc. But we only use it to check the SLC or MLC.

This patch tries to make it more clear and simple, renames the @cellinfo
to @bits_per_cell.

In order to avoiding the bisect issue, this patch also does the following
changes:
  (0) add a macro NAND_CI_CELLTYPE_SHIFT to avoid the hardcode.

  (1) add a helper to parse out the cell type : nand_get_bits_per_cell()

  (2) parse out the cell type for extended-ID chips and the full-id nand chips.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>