LoongArch: Allow to enable PREEMPT_RT

It is really time.

LoongArch has all the required architecture related changes, that have
been identified over time, in order to enable PREEMPT_RT. With the recent
printk changes, the last known road block has been addressed.

Allow to enable PREEMPT_RT on LoongArch.

Below are the latency data from cyclictest on a 4-core Loongson-3A5000
machine, with a "make -j8" kernel building workload in the background.

1. PREEMPT kernel with default configuration:

./cyclictest -a -t -m -i200 -d0 -p99
policy: fifo: loadavg: 8.78 8.96 8.64 10/296 64800
T: 0 ( 4592) P:99 I:200 C:14838617 Min:   3 Act:    6 Avg:    8 Max:    844
T: 1 ( 4593) P:99 I:200 C:14838765 Min:   3 Act:    9 Avg:    8 Max:    909
T: 2 ( 4594) P:99 I:200 C:14838510 Min:   3 Act:    7 Avg:    8 Max:    832
T: 3 ( 4595) P:99 I:200 C:14838631 Min:   3 Act:    8 Avg:    8 Max:    931

2. PREEMPT_RT kernel with default configuration:

./cyclictest -a -t -m -i200 -d0 -p99
policy: fifo: loadavg: 10.38 10.47 10.35 9/336 77788
T: 0 ( 3941) P:99 I:200 C:19439626 Min:   3 Act:   12 Avg:    8 Max:    227
T: 1 ( 3942) P:99 I:200 C:19439624 Min:   2 Act:   11 Avg:    8 Max:    184
T: 2 ( 3943) P:99 I:200 C:19439623 Min:   3 Act:    4 Avg:    7 Max:    223
T: 3 ( 3944) P:99 I:200 C:19439623 Min:   2 Act:   10 Avg:    7 Max:    226

3. PREEMPT_RT kernel with tuned configuration:

./cyclictest -a -t -m -i200 -d0 -p99
policy: fifo: loadavg: 10.52 10.66 10.62 12/334 109397
T: 0 ( 4765) P:99 I:200 C:29335186 Min:   3 Act:    6 Avg:    8 Max:     62
T: 1 ( 4766) P:99 I:200 C:29335185 Min:   3 Act:   10 Avg:    8 Max:     52
T: 2 ( 4767) P:99 I:200 C:29335184 Min:   3 Act:    8 Avg:    8 Max:     64
T: 3 ( 4768) P:99 I:200 C:29335183 Min:   3 Act:   12 Avg:    8 Max:     53

Main instruments of tuned configuration include: Disable the boot rom
space in BIOS, in order to avoid kernel's speculative access to low-
speed memory (i.e. boot rom space); Disable CPUFreq scaling; Disable
RTC synchronization in the ntpd/chronyd service (also avoid other RTC
accesses when running low-latency workloads).

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

arch/loongarch/Kconfig

@@ -66,6 +66,7 @@ config LOONGARCH
 	select ARCH_SUPPORTS_LTO_CLANG
 	select ARCH_SUPPORTS_LTO_CLANG_THIN
 	select ARCH_SUPPORTS_NUMA_BALANCING
+	select ARCH_SUPPORTS_RT
 	select ARCH_USE_BUILTIN_BSWAP
 	select ARCH_USE_CMPXCHG_LOCKREF
 	select ARCH_USE_QUEUED_RWLOCKS