dt-bindings: mtd: partitions: Fix partition node name pattern

The 'partition' node name pattern is missing start and end anchors, so
anything is allowed before or after the regex pattern. There's no in tree
users needing that, so add anchors to the pattern.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Pratyush Yadav <ptyadav@amazon.de>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230120020454.3225796-1-robh@kernel.org