mac68k: replace mac68k SCC code with platform device

Remove the old 68k Mac serial port code and a lot of related cruft. Add
new SCC platform devices to mac 68k platform.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>