[POWERPC] Fix xmon=off and cleanup xmon initialisation

My patch to make the early xmon logic work with earlier early param
parsing (480f6f35) breaks xmon=off.

No one does this obviously as xmon rocks, but it should really work
as documented.

While fixing that it struck me that we could move the xmon param
handling into xmon.c, and also consolidate the
xmon_init()/do_early_xmon logic into xmon_setup(). This means
xmon=early drops into xmon a little earlier on 32-bit, but it
seems to work just fine.

Tested on PSERIES and CLASSIC32.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>