LoongArch: Reduce min_delta for the arch clockevent device

Now the min_delta is 0x600 (1536) for LoongArch's constant clockevent
device. For a 100MHz hardware timer this means ~15us. This is a little
big, especially for PREEMPT_RT enabled kernels. So reduce it to 100 for
PREEMPT_RT kernel, and 1000 for others (we don't want too small values
to affect performance).

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>