irqchip/loongarch-cpu: Fix a missing prototype warning

1, Rename loongarch_cpu_irq_of_init() to cpuintc_of_init() in order to
   keep the same style as the ACPI version.
2, Fix a missing prototype warning by adding a "static" modifier.

Fixes: 855d4ca4 ("irqchip: loongarch-cpu: add DT support")
Reported-by: kernel test robot <lkp@intel.com>
Cc: Peibao Liu <liupeibao@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221205044708.2054022-1-chenhuacai@loongson.cn