powerpc/47x: Add prototype for mmu_init_secondary()

A W=1 build of 44x/iss476-smp_defconfig gives:

arch/powerpc/mm/nohash/44x.c:220:13: error: no previous prototype for 'mmu_init_secondary' [-Werror=missing-prototypes]
  220 | void __init mmu_init_secondary(int cpu)
      |             ^~~~~~~~~~~~~~~~~~

That function is called from head_4xx.S

Add a prototype in mmu_decl.h

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/e89d9927c926044e54fd056a849785f526c6414f.1692282340.git.christophe.leroy@csgroup.eu