LoongArch: Support PREEMPT_DYNAMIC with static keys

Since commit 4e90d052 ("riscv: support PREEMPT_DYNAMIC with
static keys"), the infrastructure is complete and we can simply select
HAVE_PREEMPT_DYNAMIC_KEY to enable PREEMPT_DYNAMIC on LoongArch because
we already support static keys.

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