clk: sunxi-ng: Check kzalloc() for errors and cleanup error path

This kzalloc() could fail. Let's bail out with -ENOMEM here
instead of NULL dereferencing. That silences static checkers. We
should also cleanup on the error path even though this function
returning an error probably means the system won't boot.

Cc: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>