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>

drivers/tty/serial/8250/Kconfig

@@ -398,8 +398,10 @@ config SERIAL_8250_INGENIC
 	  its UARTs, say Y to this option. If unsure, say N.
 
 config SERIAL_8250_MID
-	tristate "Support for serial ports on Intel MID platforms"
+	tristate "Support for serial ports on Intel MID platforms" if EXPERT
+	default SERIAL_8250
 	depends on SERIAL_8250 && PCI
+	depends on X86 || COMPILE_TEST
 	select HSU_DMA if SERIAL_8250_DMA
 	select HSU_DMA_PCI if (HSU_DMA && X86_INTEL_MID)
 	select RATIONAL