timekeeping: Increase granularity of read_persistent_clock(), build fix

Fix the following build problem on powerpc:

  arch/powerpc/kernel/time.c: In function 'read_persistent_clock':
  arch/powerpc/kernel/time.c:788: error: 'return' with a value, in function returning void
  arch/powerpc/kernel/time.c:791: error: 'return' with a value, in function returning void

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: dwalker@fifo99.com
Cc: johnstul@us.ibm.com
LKML-Reference: <20090822222313.74b9619c@skybase>
Signed-off-by: Ingo Molnar <mingo@elte.hu>