mmc: sunxi: Factor out clock phase setting code into a helper function

Add a sunxi_mmc_clk_set_phase() helper function.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>