[POWERPC] Cleanup include/asm-powerpc/xmon.h

For some reason we have two prototypes for xmon_init(), remove the
one in system.h.

No one calls xmon() anymore, debugger() is preferable, so we don't
need the prototype. And similarly no one calls xmon_printf().

Also update the include guards on xmon.h to match the standard
format, add copyright and license, and add comments to #endifs.

Built for pseries_defconfig and pmac32_defconfig.

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