schemas: chosen: allow a boot logo node

[maximpedraza0]

A boot logo supplied by firmware is configuration rather than a description of
the hardware, so it belongs under /chosen, in the same spirit as the
oem-logo variable Open Firmware carried under /options, and in the same
company as simple-framebuffer.

A Linux binding for such a node, linux,boot-logo-clut224, is being proposed
on the kernel lists:

https://lore.kernel.org/all/20260804225617.264861-1-maximpedraza@gmail.com/

chosen.yaml currently allows only ^framebuffer under /chosen, so
dtbs_check rejects the node on any board that uses it:

board.dtb: chosen: 'logo' does not match any of the regexes:
    '^framebuffer', '^pinctrl-[0-9]+$'
	from schema $id: http://devicetree.org/schemas/chosen.yaml

This allows it. The name is fixed, so it goes in properties rather than
patternProperties -- an anchored pattern there is rejected by the
metaschema, which says fixed strings belong in properties. As with
framebuffer, the contents are not validated here: the node is matched by
its own binding through its compatible string, and the kernel binding pins
the name with $nodename: const: logo.

Tested against two device trees, one carrying the image in the node and one
pointing at a reserved memory region: the error above goes from 1 to 0 on
both, with no other change in dt-validate output.