x86: allow MMCONFIG above 4GB on x86_64

SGI UV will have MMCFG base addresses that are greater than 4GB (32 bits).

v2: Use CONFIG_RESOURCES_64BIT instead of CONFIG_X86_64.
v3: Create a flag, that is set by platform specific code,
    to disable the > 4GB check.

Signed-off-by: John Keller <jpk@sgi.com>
Cc: jpk@sgi.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>