serial: 8250_mid: make module available only on X86

There is no reason to compile module on non-X86 platforms, though COMPILE_TEST
is provided for sake of what it does.

While here, set default to SERIAL_8250 that user doesn't need an explicit
option to be set and hide it from non-expert.

Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>