clk: gpio: handle error codes for of_clk_get_parent_count()

We might make bad memory allocations if we get (e.g.) -ENOSYS from
of_clk_get_parent_count().

Noticed by Coverity.

Fixes: f66541ba ("clk: gpio: Get parent clk names in of_gpio_clk_setup()")
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Jyri Sarha <jsarha@ti.com>
Cc: Sergej Sawazki <ce3a@gmx.de>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>