partition: add support for sysv68 partitions

Add support for the Motorola sysv68 disk partition (slices in motorola
doc).

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>