kbuild robot reports that s390 fails to build this driver with
COMPILE_TEST. Let's depend on HAS_IOMEM so that s390 doesn't try to
build it.

Cc: Lee Jones <lee.jones@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reported-by: kernel test robot <lkp@intel.com>
Fixes: 419b3ab6 ("clk: versatile: remove dependency on ARCH_*")
Link: https://lore.kernel.org/r/20210604192321.2594519-1-sboyd@kernel.orgSigned-off-by: Stephen Boyd <sboyd@kernel.org>