firmware: only allow EDD on x86

Rather than have the EDD depend on !ia64 (and assuming that only ia64,
x86, x86_64 will be including this Kconfig), have EDD depend on the only
arches which can support this code.  This should allow all other arches to
cleanly include the firmware Kconfig.

Also simplify the x86 string used by FIRMWARE_MEMMAP to match EDD.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Matt Domsch <Matt_Domsch@dell.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>