Special handling for MP table in low memory is only necessary if there are

Special handling for MP table in low memory is only necessary if there are
tables in low memory.

This removes a hang when HAVE_LOW_TABLES=0 and HAVE_HIGH_TABLES=1. With this
patch I can boot all the way to a payload. Tested on a Supermicro H8DME.

Many thanks to Patrick Georgi for figuring this out.

Signed-off-by: Ward Vandewege <ward@gnu.org>
Acked-by: Myles Watson <mylesgw@gmail.com>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@4220 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1