dt-bindings: mtd: fixed-partition: Add binman compatibles

Add two compatibles for binman entries, as a starting point for the
schema.

Note that, after discussion on v2, we decided to keep the existing
meaning of label so as not to require changes to existing userspace
software when moving to use binman nodes to specify the firmware
layout.

Note also that, after discussion on v6, we decided to use the same
'fixed-partition' schema for the binman features, so this version
adds a new 'binman.yaml' file providing the new compatibles to the
existing partition.yaml binding.

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/20240412153249.100787-2-sjg@chromium.org