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>