main/linux-firmware: add s5p-mfc subpackage
Add separate subpackage for Samsung S5P MFC (video decoder for some Exynos chips) firmware.
This allows us to use the firmware on mainlined devices with this MFC in postmarketOS without having to download all ~160MB of linux-firmware-other.
Related downstream MR: https://gitlab.com/postmarketOS/pmaports/-/merge_requests/4284