um: Wire CONFIG_GENERIC_IO up

UML has no io memory but implements everything defined in
generic-asm/io.h.
Signed-off-by: NRichard Weinberger <richard@nod.at>

arch/um/Kconfig.common

@@ -9,6 +9,7 @@ config UML
 	select HAVE_GENERIC_HARDIRQS
 	select GENERIC_IRQ_SHOW
 	select GENERIC_CPU_DEVICES
+	select GENERIC_IO
 
 config MMU
 	bool