MAINTAINERS: CFI flash maintainership only includes the CFI related files

All the other files in drivers/mtd/* are not really CFI related. So lets
only include the files that are CFI specific.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Jagan Teki <jagannadh.teki@gmail.com>
Acked-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Acked-by: Heiko Schocher <hs@denx.de>