Introduce CONFIG_GENERIC_IO

There are situations where CONFIG_HAS_IOMEM is too restrictive.
For example CONFIG_MTD_NAND_NANDSIM depends on CONFIG_HAS_IOMEM
but it works perfectly fine if an architecture without io memory
just includes asm-generic/io.h or implements everything defined in it.
UML is such a corner case.
Signed-off-by: Richard Weinberger <richard@nod.at>

lib/Kconfig

@@ -29,6 +29,10 @@ config GENERIC_IOMAP
 	bool
 	select GENERIC_PCI_IOMAP
 
+config GENERIC_IO
+	boolean
+	default n
+
 config CRC_CCITT
 	tristate "CRC-CCITT functions"
 	help
@@ -224,6 +228,7 @@ config BTREE
 config HAS_IOMEM
 	boolean
 	depends on !NO_IOMEM
+	select GENERIC_IO
 	default y
 
 config HAS_IOPORT