clk: mvebu: armada-37xx-periph: Remove unused var num_parents

When building armada-37xx-periph, num_parents isn't used in function
clk_pm_cpu_get_parent:
drivers/clk/mvebu/armada-37xx-periph.c: In function ‘clk_pm_cpu_get_parent’:
drivers/clk/mvebu/armada-37xx-periph.c:419:6: warning: unused variable ‘num_parents’ [-Wunused-variable]
  int num_parents = clk_hw_get_num_parents(hw);
      ^~~~~~~~~~~
Remove the declaration of num_parents to dispose the warning.

Fixes: 616bf80d ("clk: mvebu: armada-37xx-periph: Fix wrong return value in get_parent")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>