[MIPS] Sibyte: Fix M_SCD_TIMER_INIT and M_SCD_TIMER_CNT wrong field width.

    
From Dave Johnson <djohnson+linuxmips@sw.starentnetworks.com>:
    
Field width should be 23 bits not 20 bits.
    
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>