powerpc/mpic_timer: fix the time is not accurate caused by GTCRR toggle bit

When the timer GTCCR toggle bit is inverted, we calculated the rest
of the time is not accurate. So we need to ignore this bit.

Signed-off-by: Wang Dongsheng <dongsheng.wang@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>