powerpc/47x: Remove early_init_mmu_47x() to fix no previous prototype

4xx/iss476-smp_defconfig leads to:

  CC      arch/powerpc/mm/nohash/tlb.o
arch/powerpc/mm/nohash/tlb.c:322:13: error: no previous prototype for 'early_init_mmu_47x' [-Werror=missing-prototypes]
  322 | void __init early_init_mmu_47x(void)
      |             ^~~~~~~~~~~~~~~~~~

early_init_mmu_47x() is used only at one place and only locally.

Fold it into its only caller and remove it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/0a667b7c2e05d3cf41ecd38f33cc334083a61c8d.1692282396.git.christophe.leroy@csgroup.eu