clk: sunxi: add __iomem markings to MMIO pointers

This commit adds __iomem thoughout the sunxi clock driver, in places
where it was ommited. This cleans most of the sparse warnings we
are getting here.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>