dma.c defined function plat_swiotlb_setup, which is declared in
bootinfo.h.

Fixes warning:
arch/mips/loongson64/dma.c:25:13: warning: no previous prototype for ‘plat_swiotlb_setup’ [-Wmissing-prototypes]
   25 | void __init plat_swiotlb_setup(void)
      |             ^~~~~~~~~~~~~~~~~~
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>