MIPS: RB532: Declare prom_setup_cmdline() and rb532_gpio_init() static

early_tlb_init() and rb532_gpio_init() were local to file but not declared
static, leading to error:

arch/mips/rb532/prom.c:49:13: error: no previous prototype for ‘prom_setup_cmdline’ [-Werror=missing-prototypes]
   49 | void __init prom_setup_cmdline(void)
      |             ^~~~~~~~~~~~~~~~~~
arch/mips/rb532/gpio.c:200:12: error: no previous prototype for ‘rb532_gpio_init’ [-Werror=missing-prototypes]
  200 | int __init rb532_gpio_init(void)
      |            ^~~~~~~~~~~~~~~

Signed-off-by: Yongzhen Zhang <zhangyongzhen@kylinos.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>