Impact: fix boot with mptable above max_low_mapped

Try to use early_ioremap() to map MPC to make sure it works even it is
at the end of ram.
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <49AE4901.3090801@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Reported-and-tested-by: Kevin O'Connor <kevin@koconnor.net>