clk: sunxi: improve mux_clk error handling and reporting

We now catch and report a failing ioremap, also a failure in the final
step of the clock registration is now handled and reported.
Also warnings are turned into errors.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>