lib: move GENERIC_IOMAP to lib/Kconfig

define GENERIC_IOMAP in a central location
instead of all architectures. This will be helpful
for the follow-up patch which makes it select
other configs. Code is also a bit shorter this way.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/arch/ia64/Kconfig b/arch/ia64/Kconfig
index 27489b6..2732e1b 100644
--- a/arch/ia64/Kconfig
+++ b/arch/ia64/Kconfig
@@ -29,6 +29,7 @@
 	select GENERIC_IRQ_SHOW
 	select ARCH_WANT_OPTIONAL_GPIOLIB
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG
+	select GENERIC_IOMAP
 	default y
 	help
 	  The Itanium Processor Family is Intel's 64-bit successor to
@@ -102,10 +103,6 @@
 	bool
 	default y
 
-config GENERIC_IOMAP
-	bool
-	default y
-
 config ARCH_CLOCKSOURCE_DATA
 	def_bool y