sparc64: Fix prototype warning for prom_get_mmu_ihandle

arch/sparc/prom/misc_64.c:165:5: warning: no previous prototype for ‘prom_get_mmu_ihandle’

The function prom_get_mmu_ihandle has no users outside of misc_64.c so
declare it static.

Link: https://lore.kernel.org/r/20240710094155.458731-4-andreas@gaisler.com


Signed-off-by: Andreas Larsson <andreas@gaisler.com>