Remove a bunch of compilation warnings for halt/reset handlers:

arch/mips/dec/reset.c:22:17: warning: no previous prototype for 'dec_machine_restart' [-Wmissing-prototypes]
   22 | void __noreturn dec_machine_restart(char *command)
      |                 ^~~~~~~~~~~~~~~~~~~
arch/mips/dec/reset.c:27:17: warning: no previous prototype for 'dec_machine_halt' [-Wmissing-prototypes]
   27 | void __noreturn dec_machine_halt(void)
      |                 ^~~~~~~~~~~~~~~~
arch/mips/dec/reset.c:32:17: warning: no previous prototype for 'dec_machine_power_off' [-Wmissing-prototypes]
   32 | void __noreturn dec_machine_power_off(void)
      |                 ^~~~~~~~~~~~~~~~~~~~~
arch/mips/dec/reset.c:38:13: warning: no previous prototype for 'dec_intr_halt'
[-Wmissing-prototypes]
   38 | irqreturn_t dec_intr_halt(int irq, void *dev_id)
      |             ^~~~~~~~~~~~~

(which get promoted to compilation errors with CONFIG_WERROR), by moving
the local prototypes from arch/mips/dec/setup.c to a dedicated header
for arch/mips/dec/reset.c to use as well.  No functional change.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>