ARM: dts: am335x-boneblack: Add HDMI audio support

Add HDMI audio support. Adds mcasp0_pins, clk_mcasp0_fixed,
clk_mcasp0, mcasp0, sound node, and updates the tda19988 node to
follow the new binding.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>