sparc64: implement ioremap_uc

On sparc64, the whole physical IO address space is accessible using
physically addressed loads and stores. *_uc does nothing like the
others.

Cc: <stable@vger.kernel.org> # v4.19+
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Tuowen Zhao <ztuowen@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>