ia64: split off early_ioremap() declarations into asm/early_ioremap.h

Unlike x86, arm64 and ARM, ia64 does not declare its implementations
of early_ioremap/early_iounmap/early_memremap/early_memunmap in a header
file called <asm/early_ioremap.h>

This complicates the use of these functions in generic code, since the
header cannot be included directly, and we have to rely on transitive
includes, which is fragile.

So create a <asm/early_ioremap.h> for ia64, and move the existing
definitions into it.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>