lontium: create new package containing firmware for Lontium display bridge devices

Move the Lontium firmware out of firmware-misc-nonfree into a new vendor-specific package. The LT9611UXC MIPI DSI to HDMI bridge is used on a number of ARM64 development boards, particularly boards based on Qualcomm SoCs. Those systems need this firmware for display output, but have little reason to install the much broader firmware-misc-nonfree package, so this makes it easier to build minimal, installer, cloud and embedded images with a clearly defined firmware set.

The bridge is an external component rather than part of the SoC and may also be used with platforms from other SoC vendors, so a vendor-specific package is preferable to adding the files to firmware-qcom-soc.

All three patterns have to move together, since lt8713sx_fw.bin and lt9611uxc_fw.bin are upstream symlinks into the Lontium subdirectory.

firmware-misc-nonfree recommends firmware-lontium so that this firmware remains installed when upgrading; as with the packages split out in 20230625-3~exp1, this can be demoted to Suggests after the next stable release.

Closes: Bug#1143446