Load an IDT with NULL limit

Load an IDT with NULL limit

Load an IDT with NULL limit to prevent the 16bit IDT being used
in protected mode before c_start.S sets up a 32bit IDT when entering
ram stage.

Signed-off-by: Stefan Reinauer <reinauer@google.com>
Change-Id: I8d048c894c863ac4971fcef8f065be6b899e1d3e
Reviewed-on: http://review.coreboot.org/259
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>