powerpc/mm: enable page parallel initialisation

Parallel initialisation has been enabled for X86, boot time is improved
greatly. On Power8, it is improved greatly for small memory. Here is
the result from my test on Power8 platform:

For 4GB of memory, boot time is improved by 59%, from 24.5s to 10s.

For 50GB memory, boot time is improved by 22%, from 56.8s to 43.8s.

Signed-off-by: Li Zhang <[e-mail]>
Acked-by: Mel Gorman <[e-mail]>
Acked-by: Michael Ellerman <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/powerpc/Kconfig

···
 	select ARCH_HAS_DEVMEM_IS_ALLOWED
 	select HAVE_ARCH_SECCOMP_FILTER
 	select ARCH_HAS_UBSAN_SANITIZE_ALL
+	select ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT
 
 config GENERIC_CSUM
 	def_bool CPU_LITTLE_ENDIAN