MIPS: lantiq: irq: fix misc missing-prototypes warnings

Fix the following build warnings:

arch/mips/lantiq/irq.c:340:12: error: no previous prototype for 'icu_of_init' [-Werror=missing-prototypes]
  340 | int __init icu_of_init(struct device_node *node, struct device_node *parent)
      |            ^~~~~~~~~~~
arch/mips/lantiq/irq.c:418:5: error: no previous prototype for 'get_c0_perfcount_int' [-Werror=missing-prototypes]
  418 | int get_c0_perfcount_int(void)
      |     ^~~~~~~~~~~~~~~~~~~~
arch/mips/lantiq/irq.c:424:14: error: no previous prototype for 'get_c0_compare_int' [-Werror=missing-prototypes]
  424 | unsigned int get_c0_compare_int(void)
      |              ^~~~~~~~~~~~~~~~~~

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>