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/x86/Kconfig b/arch/x86/Kconfig
index cb9a104..08af645 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -75,6 +75,7 @@
 	select HAVE_BPF_JIT if (X86_64 && NET)
 	select CLKEVT_I8253
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG
+	select GENERIC_IOMAP
 
 config INSTRUCTION_DECODER
 	def_bool (KPROBES || PERF_EVENTS)
@@ -140,9 +141,6 @@
 config GENERIC_ISA_DMA
 	def_bool ISA_DMA_API
 
-config GENERIC_IOMAP
-	def_bool y
-
 config GENERIC_BUG
 	def_bool y
 	depends on BUG