mtd: spi-nor: introduce Octo SPI protocols

This patch starts adding support to Octo SPI protocols (SPI x-y-8).

Op codes for Fast Read and/or Page Program operations using Octo SPI
protocols are not known yet (no JEDEC specification has defined them yet)
but we'd rather introduce the Octo SPI protocols now so it's done as it
should be.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>