ptp_qoriq: fix latency in ptp_qoriq_adjtime() operation

ptp_qoriq: fix latency in ptp_qoriq_adjtime() operation

1588 driver loses about 1us in adjtime operation at PTP slave
This is because adjtime operation uses a slow non-atomic tmr_cnt_read()
followed by tmr_cnt_write() operation.

In the above sequence, since the timer counter operation keeps
incrementing, it leads to latency. The tmr_offset register
(which is added to TMR_CNT_H/L register giving the current time)
must be programmed with the delta nanoseconds.

Signed-off-by: Nikhil Gupta <nikhil.gupta@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20230119204034.7969-1-nikhil.gupta@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>