arm64: mark split_pmd() with __init to avoid section mismatch warnings

split_pmd() calls early_alloc(), which is marked with __init. Mark
split_pmd() similarly. The only current caller of split_pmd() is
remap_pages(), which is already __init, so there was no real danger
here in the first place.

Change-Id: I3bbc4c66f1ced8fe772366b7e5287be5f474f314
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>