mtd: nand_ids: introduce helper macros

Introduce helper macros for defining NAND chips. These macros do not really add
much value in the current code-base. However, we are going to add full ID
support which adds some more complexity to the table, and helper macros become
useful for readability.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>