powerpc: Move prom_init() out of asm-prototypes.h

This is the end of the work started with commit 76222808 ("powerpc:
Move C prototypes out of asm-prototypes.h")

Now that asm/machdep.h doesn't include asm/setup.h anymore, there are
no conflicts anymore with the function prom_init() defined in
drivers/gpu/drm/nouveau/nvkm/subdev/bios/shadowrom.o

So we can move it to asm/setup.h

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e111e4f0addb0fa810d5f6a71d3b8e62c0b53492.1654966508.git.christophe.leroy@csgroup.eu