dt-bindings: mtd: st,spear600-smi: Fix description

dt-bindings: mtd: st,spear600-smi: Fix description

The description mixes two nodes. There is the controller, and there is
the flash. Describe the flash (which itself can be considered an mtd
device, unlike the top level controller), and move the st,smi-fast-mode
property inside, as this property is flash specific and should not live
in the parent controller node.

Fixes: 68cd8ef48452 ("dt-bindings: mtd: st,spear600-smi: convert to DT schema")
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>