soc: rockchip: power-domain: Add a sanity check on pd->num_clks

The of_count_phandle_with_args() can fail and return error(for example,
rk3399 pd_vio doesn't have clocks). That would break the pd probe.

Add a sanity check on pd->num_clks to avoid that.

Fixes: 65084121d59d ("soc: rockchip: power-domain: use clk_bulk APIs")
Reported-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Tested-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>