LoongArch: Remove MIPS comment about cycle counter

This comment block was taken originally from the MIPS architecture code,
where indeed there are particular assumptions one can make regarding SMP
and !SMP and cycle counters. On LoongArch, however, the rdtime family of
functions is always available. As Xuerui wrote:

The rdtime family of instructions is in fact guaranteed to be
available on LoongArch; LoongArch's subsets all contain them, even
the 32-bit "Primary" subset intended for university teaching -- they
provide the rdtimeh.w and rdtimel.w pair of instructions that access
the same 64-bit counter.

So this commit simply removes the incorrect comment block.

Link: https://lore.kernel.org/lkml/e78940bc-9be2-2fe7-026f-9e64a1416c9f@xen0n.name/
Fixes: b738c106f735 ("LoongArch: Add other common headers")
Reviewed-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Add other common headers

Add some other common headers for basic LoongArch support.

Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>