irqchip/loongson-pch-pic: Move find_pch_pic() into CONFIG_ACPI

MIPS doesn't declare find_pch_pic(), which makes a build warning:

>> drivers/irqchip/irq-loongson-pch-pic.c:51:5: warning: no previous prototype for function 'find_pch_pic' [-Wmissing-prototypes]
   int find_pch_pic(u32 gsi)
       ^
   drivers/irqchip/irq-loongson-pch-pic.c:51:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
   int find_pch_pic(u32 gsi)
   ^
   static
   1 warning generated.

Move find_pch_pic() into CONFIG_ACPI which only used by LoongArch to fix
the warning.

BTW, remove the duplicated declaration of find_pch_pic() in irq.h.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220808093205.3658485-1-chenhuacai@loongson.cn