x86: cleanup machine_specific_memory_setup, v2

1. let 64bit support 88 and e801 too
2. introduce default_machine_specific_memory_setup, and reuse it
   for voyager

v2: fix 64 bit compiling

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>