Documentation: mtd: update the document for m25p80

The m25p80.c has used the SPI NOR framework now, and the m25p_ids has
been moved to spi-nor.c and renamed to spi_nor_ids.

This patch updates the document for m25p80.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>