kernel: switch compatible property for RedBoot DT binding

kernel: switch compatible property for RedBoot DT binding

This changes the DT binding's compatible property to
"ecoscentric,redboot-fis-partitions", removing the existing reference to
Red Hat.

Per the documentation hosted at eCosCentric's website, eCosCentric is
RedBoot's sole commercial maintainer since 2002, and the project has
been under the stewardship of the Free Software Foundation since 2008.

This also updates the property in the Inventel Livebox 1 .dts, the
binding's only current user.

Signed-off-by: Matt Merhar <mattmerhar@protonmail.com>
(backported from a2eaaaaf4b88f8e917203d79d833a9686a61022f)