arc: add stubs for map_physmem() and unmap_physmem()

Up until now there was no need in those stubs.

But since following commit compilation of U-Boot on ARC is broken:
------------------------>8----------------------
commit 7861204c
Author: Stephen Warren <swarren@wwwdotorg.org>
Date:   Sat Oct 3 13:56:46 2015 -0600

    itest: make memory access work under sandbox

    itest accesses memory, and hence must map/unmap it. Without doing so, it
    accesses invalid addresses and crashes.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
------------------------>8----------------------

That's because CMD_ITEST is enabled by default in common/Kconfig and now
map_physmem()/unmap_physmem() is used there.

So this patch adds missing stubs for ARC.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Stephen Warren <swarren@wwwdotorg.org>

arch/arc/include/asm/io.h

@@ -10,6 +10,30 @@
 #include <linux/types.h>
 #include <asm/byteorder.h>
 
+/*
+ * Given a physical address and a length, return a virtual address
+ * that can be used to access the memory range with the caching
+ * properties specified by "flags".
+ */
+#define MAP_NOCACHE	(0)
+#define MAP_WRCOMBINE	(0)
+#define MAP_WRBACK	(0)
+#define MAP_WRTHROUGH	(0)
+
+static inline void *
+map_physmem(phys_addr_t paddr, unsigned long len, unsigned long flags)
+{
+	return (void *)((unsigned long)paddr);
+}
+
+/*
+ * Take down a mapping set up by map_physmem().
+ */
+static inline void unmap_physmem(void *vaddr, unsigned long flags)
+{
+
+}
+
 static inline void sync(void)
 {
 	/* Not yet implemented */