metag: Add JTAG Debug Adapter (DA) support

Add basic JTAG Debug Adapter (DA) support so that drivers which
communicate with the DA can detect whether one is actually present
(otherwise the target will halt indefinitely).

This allows the metag_da TTY driver and imgdafs filesystem driver to be
built, updates defconfigs, and sets up the metag_da console early if
it's configured in.

Signed-off-by: James Hogan <james.hogan@imgtec.com>