Fix the following missing-prototypes build warning:
arch/mips/lantiq/xway/dcdc.c:49:12: error: no previous prototype for 'dcdc_init' [-Werror=missing-prototypes]
49 | int __init dcdc_init(void)
| ^~~~~~~~~
Signed-off-by: Shiji Yang <yangshiji66@outlook.com> Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>