MIPS: lantiq: falcon: fix misc missing-prototypes warnings

Fix the following build warnings:

arch/mips/lantiq/falcon/prom.c:39:13: error: no previous prototype for 'ltq_soc_nmi_setup' [-Werror=missing-prototypes]
   39 | void __init ltq_soc_nmi_setup(void)
      |             ^~~~~~~~~~~~~~~~~
arch/mips/lantiq/falcon/prom.c:46:13: error: no previous prototype for 'ltq_soc_ejtag_setup' [-Werror=missing-prototypes]
   46 | void __init ltq_soc_ejtag_setup(void)
      |             ^~~~~~~~~~~~~~~~~~~

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>