serial: lantiq: Add x86 in Kconfig dependencies for Lantiq serial driver

Lantiq serial driver/IP is reused for a x86 based SoC as well.
Update the Kconfig accordingly.

Signed-off-by: Rahul Tanwar <rahul.tanwar@linux.intel.com>
Link: https://lore.kernel.org/r/96fd193c0a8939d27641ff93573545c02313048f.1588577002.git.rahul.tanwar@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>