x86: make e820_end return end_of_ram again for 64bit

even on 64bit systems with less than 4G RAM, we can now use fixmap
to handle acpi SIT near end of ram.

change e820_end to e820_end_of_ram again?
or e820_ram_pfn?

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>