clk: sunxi: Make the mod0 clk driver also a platform driver

With the prcm in sun6i (and some later SoCs) some mod0 clocks are instantiated
through the mfd framework, and as such do not work with of_clk_declare, since
they do not have registers assigned to them yet at of_clk_declare init time.

Silence the error on not finding registers in the of_clk_declare mod0 clk
setup method, and also register mod0-clk support as a platform driver to work
properly with mfd instantiated mod0 clocks.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>