sh: machvec rework.

Some more machvec overhauling and setup code cleanup. Kill off
get_system_type() and platform_setup(), we can do these both
through the machvec. While we're add it, kill off more useless
mach.c's and drop some legacy cruft from setup.c.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>