lib: add iomem emulation (logic_iomem)

Add IO memory emulation that uses callbacks for read/write to
the allocated regions. The callbacks can be registered by the
users using logic_iomem_alloc().

To use, an architecture must 'select LOGIC_IOMEM' in Kconfig
and then include <asm-generic/logic_io.h> into asm/io.h to get
the __raw_read*/__raw_write* functions.

Optionally, an architecture may 'select LOGIC_IOMEM_FALLBACK'
in which case non-emulated regions will 'fall back' to the
various real_* functions that must then be provided.

Cc: Arnd Bergmann <arnd@kernel.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

lib/Kconfig

@@ -102,6 +102,20 @@ config INDIRECT_PIO
 
 	  When in doubt, say N.
 
+config INDIRECT_IOMEM
+	bool
+	help
+	  This is selected by other options/architectures to provide the
+	  emulated iomem accessors.
+
+config INDIRECT_IOMEM_FALLBACK
+	bool
+	depends on INDIRECT_IOMEM
+	help
+	  If INDIRECT_IOMEM is selected, this enables falling back to plain
+	  mmio accesses when the IO memory address is not a registered
+	  emulated region.
+
 config CRC_CCITT
 	tristate "CRC-CCITT functions"
 	help