mflash: Initial mflash support

Mflash is fusion memory device mainly targeted consumer eletronic and
mobile phone.
Internally, it have nand flash and other hardware logics and supports
some different operation (ATA, IO, XIP) modes.

IO mode is custom mode for the host that doesn't have IDE interface.
(Many mobile targeted SoC doesn't have IDE bus)

This driver support mflash IO mode.

Followings are brief descriptions about IO mode.

1. IO mode based on ATA protocol and uses some custom command. (read
   confirm, write confirm)
2. IO mode uses SRAM bus interface.

Signed-off-by: unsik Kim <donari75@gmail.com>