nds32: ag101/ag102: Fix setting lastdec and now values

The timer3 counter unit for lastdesc and now values are inconsistent in current
code. The unit of "readl(&tmr->timer3_counter) / (CONFIG_SYS_CLK_FREQ / 2)" is
second. However, CONFIG_SYS_HZ is defined as 1000 in board config file.
This means the accuracy of "lastdec" and "now" should be in millisecond,
thus fix the equation to set lastdec and now variables accordingly.

Signed-off-by: Axel Lin <axel.lin@ingics.com>