ARM: mvebu: add "spi-flash" compatible string

U-boots spi-nor support is currently considered a work in progress. For
now to avoid issues it is necessary to add a "spi-flash" compatible
string. Eventually the "jedec,spi-nor" will be sufficient when the core
U-boot code is updated to support it.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Signed-off-by: Stefan Roese <sr@denx.de>