early_init() and machine_init() have no prototype, add one in
asm-prototypes.h.

Fixes the following warnings (treated as error in W=1):
  arch/powerpc/kernel/setup_32.c:68:30: error: no previous prototype for ‘early_init’
  arch/powerpc/kernel/setup_32.c:99:21: error: no previous prototype for ‘machine_init’
Signed-off-by: Mathieu Malaterre <malat@debian.org>
[mpe: Move them to asm-prototypes.h, drop other functions]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>