linux-wandboard

powerpc/powermac: Fixup default serial port device for pmac_zilog

This removes the non-working code in legacy_serial that tried to handle
the powermac SCC ports, and instead add a (now working) function to the
powermac platform code to find the default serial console if any.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>