AtomFpga: Add support for HDMI Audio

[hoglet67]

See discussion in this post: https://stardot.org.uk/forums/viewtopic.php?p=410514#p410514

Update the AtomFPGA HDMI code to add audio

At the moment AtomFPGA is strictly DVI (@60Hz) but BeebFPGA is HDMI, using this for the HDMI metadata: https://github.com/hoglet67/BeebFpga/blob/master/src/common/hdmi/hdmidataencoder.v

This is likely non-trivial!