dt-bindings: mtd: fixed-partitions: Fix 'sercomm,scpart-id' schema

The schema for 'sercomm,scpart-id' is broken. The 'if' condition is
never true because 'compatible' is in the parent node, not the child
node the sub-schema applies to. The example passes as there are no
constraints on additional/unevaluated properties. That's a secondary
issue which is complicated due to nested partitions.

Drop the if/then schema and the unnecessary 'allOf' so that the
'sercomm,scpart-id' property is at least defined.

Cc: Mikhail Zhilkin <csharper2005@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221111212824.4103514-1-robh@kernel.org