Revert "Disable the 8042 MUX on our x86 devices." This reverts commit 4deda9c24f1deb015946c709d1d3d11f1c0e2325. BUG=chrome-os-partner:2811, chromium-os:13101 TEST=boot and verify /proc/cmdline TBR=puneetster Change-Id: I52990abc01fe5fc2d160d8efbb0a14bff1943ae4 (cherry picked from commit 1649ba8c1be54ca34355198ab3808a0d7480c484)