clk: sunxi: Improve divs_clk error handling and reporting

We catch errors in the base clock registration, failure to ioremap
and failures in the final of_clk_add_provider() call.
Also we unmap the registers when we need to rollback.

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