BeagleBone DTS : Enable run-time pinmux for HDMI

Add cape-universal-hdmi pin info to am335x-bone-common-pinmux.dtsi
Edit hdmi dtsi include files to use new mode= setting to set HDMI
mode at startup, leaving pinmux configurable at runtime.

Signed-off-by: Charles Steinkuehler <charles@steinkuehler.net>

No-audio version works as expected, but HDMI with audio seems to not bring up the display properly.  I think this might have to do with being built as modules instead of being compiled directly into the kernel.

It's easy enough to switch back and forth between the two pinmux methods, just turn all the mode = "hdmi" into status = "disabled" in the nxp-hdmi include files, the pinmux additions can be left in place.