mtd: spi-nor: Add ISSI is25lp080d support

Add support for a new ISSI 1MB SPI NOR chip that was tested in our lab.
Datasheet is available at: http://www.issi.com/WW/pdf/25LP-WP080D.pdf

Testing was done only without the SPI_NOR_{DUAL,QUAD}_READ flags that
were added later, according to the datasheet.

Tested-by: Pascal Fabreges <pascal.fabreges@nokia.com>
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Romain Porte <romain.porte@nokia.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>