DynamicTablesPkg: add package .dsc file
    
    Having a top-level .dsc makes it easier to perform standalone build
    tests of the core code, so add one.
    
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
    Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>