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>

arch/loongarch/Kconfig

@@ -135,6 +135,7 @@ config LOONGARCH
 	select HAVE_PERF_EVENTS
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
+	select HAVE_PREEMPT_DYNAMIC_KEY
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RETHOOK
 	select HAVE_RSEQ