powerpc: Re-enable ARCH_ENABLE_SPLIT_PMD_PTLOCK

Commit 66f24fa7 ("mm: drop redundant ARCH_ENABLE_SPLIT_PMD_PTLOCK")
broke PMD split page table lock for powerpc.

It selects the non-existent config ARCH_ENABLE_PMD_SPLIT_PTLOCK in
arch/powerpc/platforms/Kconfig.cputype, but clearly intended to
select ARCH_ENABLE_SPLIT_PMD_PTLOCK (notice the word swapping!), as
that commit did for all other architectures.

Fix it by selecting the correct symbol ARCH_ENABLE_SPLIT_PMD_PTLOCK.

Fixes: 66f24fa7

 ("mm: drop redundant ARCH_ENABLE_SPLIT_PMD_PTLOCK")
Cc: stable@vger.kernel.org # v5.13+
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
[mpe: Reword change log to make it clear this is a bug fix]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210819113954.17515-3-lukas.bulwahn@gmail.com