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

drivers/clk/rockchip/clk-pll.c

@@ -94,6 +94,11 @@ static int rockchip_pll_wait_lock(struct rockchip_clk_pll *pll)
 	unsigned int val;
 	int delay = 24000000, ret;
 
+	if (IS_ERR(grf)) {
+		pr_err("%s: grf regmap not available\n", __func__);
+		return PTR_ERR(grf);
+	}
+
 	while (delay > 0) {
 		ret = regmap_read(grf, pll->lock_offset, &val);
 		if (ret) {