arch/ia64: Define early_memunmap()

This is odd to use early_iounmap() function do tear down mapping
created by early_memremap() function, even if it works right now,
because they belong to different set of functions. The former is
I/O related function and the later is memory related. So, create
early_memunmap() macro which in real is early_iounmap(). This
thing will help to not confuse code readers longer by mixing
functions from different classes.

EFI patches following this patch uses that functionality.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>