ACPI: Overriding ACPI tables via initrd only works with an initrd and on X86

ACPI: Overriding ACPI tables via initrd only works with an initrd and on X86

Reflect this dependency in Kconfig, to prevent build failures.

Shorten the config description as suggested by Borislav Petkov.

Finding a suitable memory area to store the modified table(s) has been
taken over from arch/x86/kernel/setup.c and makes use of max_low_pfn_mapped:
memblock_find_in_range(0, max_low_pfn_mapped,...)
This one is X86 specific. It may not be hard to extend this functionality
for other ACPI aware architectures if there is need for.

For now make this feature only available for X86 to avoid build failures on
IA64, compare with:
https://bugzilla.kernel.org/show_bug.cgi?id=54091

Signed-off-by: Thomas Renninger <trenn@suse.de>
Link: http://lkml.kernel.org/r/1361538742-67599-3-git-send-email-trenn@suse.de
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>