MIPS: loongson64: make CPU_LOONGSON64 depends on MIPS_FP_SUPPORT

mach/loongson64 fails to build when the FPU support is disabled:

arch/mips/loongson64/cop2-ex.c:45:15: error: implicit declaration of function ‘__is_fpu_owner’; did you mean ‘is_fpu_owner’? [-Werror=implicit-function-declaration]
arch/mips/loongson64/cop2-ex.c:98:30: error: ‘struct thread_struct’ has no member named ‘fpu’
arch/mips/loongson64/cop2-ex.c:99:30: error: ‘struct thread_struct’ has no member named ‘fpu’
arch/mips/loongson64/cop2-ex.c:131:43: error: ‘struct thread_struct’ has no member named ‘fpu’
arch/mips/loongson64/cop2-ex.c:137:38: error: ‘struct thread_struct’ has no member named ‘fpu’
arch/mips/loongson64/cop2-ex.c:203:30: error: ‘struct thread_struct’ has no member named ‘fpu’
arch/mips/loongson64/cop2-ex.c:219:30: error: ‘struct thread_struct’ has no member named ‘fpu’
arch/mips/loongson64/cop2-ex.c:283:38: error: ‘struct thread_struct’ has no member named ‘fpu’
arch/mips/loongson64/cop2-ex.c:301:38: error: ‘struct thread_struct’ has no member named ‘fpu’

Fixes: ef2f826c

 ("MIPS: Loongson-3: Enable the COP2 usage")
Suggested-by: Huacai Chen <chenhuacai@kernel.org>
Reviewed-by: Huacai Chen <chenhuacai@kernel.org>
Reported-by: k2ci robot <kernel-bot@kylinos.cn>
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>