irqchip: Fix "Loongson HyperTransport Vector support" driver build on all non-MIPS platforms

This commit:

  818e915f: ("irqchip: Add Loongson HyperTransport Vector support")

Added a MIPS-only driver, but turned on compilation on all other architectures as well:

 config LOONGSON_HTVEC
        bool "Loongson3 HyperTransport Interrupt Vector Controller"
        depends on MACH_LOONGSON64 || COMPILE_TEST

But this driver was never build tested on any other architecture than MIPS:

  drivers/irqchip/irq-loongson-htvec.c: In function ‘htvec_irq_dispatch’:
  drivers/irqchip/irq-loongson-htvec.c:59:3: error: implicit declaration of function ‘spurious_interrupt’; did you mean ‘smp_reboot_interrupt’? [-Werror=implicit-function-declaration]

Because spurious_interrupt() only exists on MIPS.

So make it MIPS-only.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>