mtd: spi-nor: add support for Micron M25PX80

This commit adds the support in the spi-nor driver of the Micron
M25PX80 flash, a 8 Mbit SPI flash from Micron.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>