m68k: sun3: Descend to prom from arch/m68k/sun3

Move prom/ to the more relevant Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20200526123810.301667-2-masahiroy@kernel.org


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>