LoongArch: Add HOTPLUG_SMT implementation

LoongArch: Add HOTPLUG_SMT implementation

For benchmarking or debugging purpose, we usually want to control SMT
via boot parameter and sysfs knobs. So add HOTPLUG_SMT implementation.

1. Boot parameters:

nosmt:		Disable SMT, can be enabled via sysfs knobs.
nosmt=force:	Disable SMT, cannot be enabled via sysfs knobs.

2. Runtime sysfs controls:

Write "on", "off", "forceoff" or the number of SMT threads (1, 2, ...)
to /sys/devices/system/cpu/smt/control.

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