[DRIVERS] clk: Minor bug fix in _clk_register()

The clk->num_parents can be 0 for clocks with no parent clocks
so clk->parent_names can very well be NULL for such clocks.

We should only check clk->parent_names to be not NULL for
clk->num_parents > 0.

Signed-off-by: Anup Patel <anup@brainfault.org>