powerpc/64s: Fix early_init_mmu section mismatch

Christian reports:

  MODPOST vmlinux.o
  WARNING: modpost: vmlinux.o(.text.unlikely+0x1a0): Section mismatch in
  reference from the function .early_init_mmu() to the function
  .init.text:.radix__early_init_mmu()
  The function .early_init_mmu() references
  the function __init .radix__early_init_mmu().
  This is often because .early_init_mmu lacks a __init
  annotation or the annotation of .radix__early_init_mmu is wrong.

  WARNING: modpost: vmlinux.o(.text.unlikely+0x1ac): Section mismatch in
  reference from the function .early_init_mmu() to the function
  .init.text:.hash__early_init_mmu()
  The function .early_init_mmu() references
  the function __init .hash__early_init_mmu().
  This is often because .early_init_mmu lacks a __init
  annotation or the annotation of .hash__early_init_mmu is wrong.

The compiler is uninlining early_init_mmu and not putting it in an init
section because there is no annotation. Add it.

Reported-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Link: https://lore.kernel.org/r/20200429070247.1678172-1-npiggin@gmail.com