MIPS: BCM47XX: Declare early_tlb_init() static

early_tlb_init() was local to file but not declared static,
leading to error:

arch/mips/bcm47xx/prom.c:126:6: error: no previous prototype for ‘early_tlb_init’ [-Werror=missing-prototypes]
  126 | void early_tlb_init(void)
      |      ^~~~~~~~~~~~~~

Signed-off-by: Yongzhen Zhang <zhangyongzhen@kylinos.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>