clk: check parent_name in clk_register to avoid confusing log_error() output

For some gate clocks and fixed clocks without a parent, calling
clk_register will print an useless error message indicating that parent
is missing. Fix that by gaurding log_xxx() with an if-statement.

Signed-off-by: Yang Xiwen <forbidden405@outlook.com>
Suggested-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Link: https://lore.kernel.org/r/20230807-clk-fix-v2-1-0b688e21fb4e@outlook.com