Documentation: add the document for the SPI NOR framework

                          SPI NOR framework

               ============================================

Part I - why we need this framework?

-------------------------------------

The SPI bus controller only deals with the byte stream.

Some controller does not works like a SPI bus controller, it works

like a SPI NOR controller instead, such as the Freescale's QuadSPI controller.

The Freescale's QuadSPI controller should know the NOR commands to

find the right LUT sequence. Unfortunately, the old code can not meet

this requirement.

Part II - How does the framework work?

-------------------------------------

This framework just adds a new layer between the MTD and the SPI bus driver.

With this new layer, the SPI NOR controller driver does not depend on the

m25p80 code anymore.

   Before this framework, the layer is like:

                   MTD

         ------------------------

                  m25p80

         ------------------------

	       SPI bus driver

         ------------------------

	        SPI NOR chip

   After this framework, the layer is like:

                   MTD

         ------------------------

              SPI NOR framework

         ------------------------

                  m25p80

         ------------------------

	       SPI bus driver

         ------------------------

	       SPI NOR chip

  With the SPI NOR controller driver(Freescale QuadSPI), it looks like:

                   MTD

         ------------------------

              SPI NOR framework

         ------------------------

                fsl-quadSPI

         ------------------------

	       SPI NOR chip

Part III - How can the drivers use the framework

-------------------------------------

The main API is the spi_nor_scan(). Before you call the hook, you should

initialize the necessary fields for spi_nor{}.

Please see the drivers/mtd/spi-nor/spi-nor.c for detail.

Please also reference to the fsl-quadspi.c when you want to write a new driver

for a SPI NOR controller.