Masking/unmasking the CPU UART irq in CP0_Status (and redirecting it to
other CPUs) may cause interrupts be lost, especially in multi-package
machines (Package-0's UART irq cannot be delivered to others). So make
mask_loongson_irq() and unmask_loongson_irq() be no-ops.

The original problem (UART IRQ may deliver to any core) is also because
of masking/unmasking the CPU UART irq in CP0_Status. So it is safe to
remove all of the stuff.
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20433/
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Cc: Huacai Chen <chenhuacai@gmail.com>