dt-bindings: mtd: fixed-partitions: Add alignment properties

Add three properties for controlling alignment of partitions, aka
'entries' in fixed-partition.

For now there is no explicit mention of hierarchy, so a 'section' is
just the 'fixed-partitions' node.

These new properties are inputs to the Binman packaging process, but are
also needed if the firmware is repacked, to ensure that alignment
constraints are not violated. Therefore they are provided as part of
the schema.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240412153249.100787-1-sjg@chromium.org