clk: rockchip: check grf when waiting pll lock

rockchip_clk_get_grf pass on return value from
syscon_regmap_lookup_by_phandle, so we check grf to
make sure whether to do the following things or not.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>