clk: imx: pllv3: fix potential 'divide by zero' in sys_get_rate()

Guard 'parent_rate==0' to prevent 'divide by zero' issue in
clk_pplv3_sys_get_rate(). If it is 0, let's return with -EINVAL.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>