The 'arm,vexpress-flash' compatible is in use, but has never been documented,
so add it now.

Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: NRob Herring <robh@kernel.org>
Acked-by: NMiquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20210819182427.1175753-1-robh@kernel.orgSigned-off-by: NRob Herring <robh@kernel.org>