x86: mention ACPI in top-level Kconfig menu

Impact: clarify menuconfig text

Mention ACPI in the top-level menu to give a clue as to where
it lives. This matches what ia64 does.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>