mn10300: Switch do_timer() to xtimer_update()

Only one CPU gets the timer interrupt so mn10300_last_tsc does not
need to be protected by xtime lock. Remove xtime lovking and use
xtime_update() which does the locking itself.

Signed-off-by: Torben Hohn <torbenh@gmx.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Koichi Yasutake <yasutake.koichi@jp.panasonic.com>
LKML-Reference: <20110127150011.23248.62040.stgit@localhost>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>