sparc32: Remove superfluous extern declarations for prom_*() functions

prom_meminit() and prom_ranges_init() are declared in <asm/oplib_32.h>.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>