MIPS: BCM47XX: include header for bcm47xx_prom_highmem_init() prototype

bcm47xx_prom_highmem_init() is a global function declared in
arch/mips/bcm47xx/bcm47xx_private.h, but this header is not
included before the definition, causing a error:

arch/mips/bcm47xx/prom.c:134:13: error: no previous prototype for ‘bcm47xx_prom_highmem_init’ [-Werror=missing-prototypes]
  134 | void __init bcm47xx_prom_highmem_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>