Define the contents of VBT block 55 (RGB Palette Table).

Note that I've not actually seen any real world VBTs with this
block.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240503122449.27266-31-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>