mtd: nand_ids: improve LEGACY_ID_NAND macro a bit

Notice that all the flashes belonging to the "legacy ID" class have 512 bytes
NAND page. This means we may simplify the 'LEGACY_ID_NAND()' macro as well as
the NAND ID table a little.

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