uml: fix bad NTP interaction with clock

UML's supposed nanosecond clock interacts badly with NTP when NTP
decides that the clock has drifted ahead and needs to be slowed down.
Slowing down the clock is done by decrementing the cycle-to-nanosecond
multiplier, which is 1.  Decrementing that gives you 0 and time is
stopped.

This is fixed by switching to a microsecond clock, with a multiplier
of 1000.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>