had a quick browse through the dts, not as easy to read as the source files in the kernel! Anyway you seem to have done everything I did via uboot, so surprising that hdmi came up.
What do the status files say in /proc/device-tree for lcdc and mcasp?
Modern uboot reads the beagle device name from the eeprom, so you don't even get the chance to specify! Think uboot must have a specific beagle compile option, that gives a specific beagle* uboot ...
Oh yes, did you recompile the dts file to am335x-boneblack.dtb?