Fix the following warning:

  CC      arch/mips/ralink/timer-gic.o
arch/mips/ralink/timer-gic.c:18:13: warning: no previous prototype for 'plat_time_init' [-Wmissing-prototypes]
   18 | void __init plat_time_init(void)
      |             ^~~~~~~~~~~~~~
Signed-off-by: Vincent Legoll <vincent.legoll@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>