x86: Add two helper macros for fixed address mapping

Sometimes fixmap will be used to map an physical address which
is not PAGE align, so to use it we need first map it and then
add the address offset to the mapped fixed address. These 2 new
helpers are suggested by Ingo Molnar to make the process
simpler.

For a physicall address like "phys", a directly usable virtual
address can be get by
	virt = (void *)set_fixmap_offset(fixed_idx, phys);
or
	virt = (void *)set_fixmap_offset_nocache(fixed_idx, phys);
(depends on whether the physical address is cachable or not).

Signed-off-by: Feng Tang <feng.tang@intel.com>
Cc: alan@linux.intel.com
Cc: greg@kroah.com
Cc: x86@kernel.org
LKML-Reference: <1284361736-23011-3-git-send-email-feng.tang@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>