ARM: add support for generic early_ioremap/early_memremap

This enables the generic early_ioremap implementation for ARM.

It uses the fixmap region reserved for kmap. Since early_ioremap
is only supported before paging_init(), and kmap is only supported
afterwards, this is guaranteed not to cause any clashes.

Tested-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>