x86: Allow use of simple_uart driver with ia32 target

Needed to get simple uart driver working in qemu with
SLIP networking or qemu<->qemu network testing. Because
this is only needed in qemu the patch only touches ia32
platform.

This can conflict with Bluetooth in x86 and qemu so caveat emptor.

Change-Id: Iba20543f968c8fd37ee36747d9aa1ad3f782057d
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>