x86: let MPS support be selectable, v2

v2: seperate "fix for compiling when MPPARSE is not set" to another patch
    make X86_MPPARSE to be selectable only when acpi is set and
    X86_MPPARSE will be set if acpi is not set.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>