mtd: spi-nor: Properly set SECT_4K for recently added flashes

Few recently added entries are missing SECT_4K flag despite of these
flashes supporting 4 KiB erase sectors and 0x20 erase command.
Also add a comment to help avoiding such mistakes in the future.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: Knut Wohlrab <knut.wohlrab@de.bosch.com>
Cc: Huang Shijie <shijie.huang@intel.com>
Cc: Shengzhou Liu <shengzhou.liu@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>