CLK: ARC: HSDK: make set_clock optional

We don't want to allow change some clocks, i.e. DDR clock.
So allow to have set_clock to be unset in clock map.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
diff --git a/drivers/clk/clk-hsdk-cgu.c b/drivers/clk/clk-hsdk-cgu.c
index 6aba458..dd665b1 100644
--- a/drivers/clk/clk-hsdk-cgu.c
+++ b/drivers/clk/clk-hsdk-cgu.c
@@ -662,7 +662,10 @@
 	if (hsdk_prepare_clock_tree_branch(sclk))
 		return -EINVAL;
 
-	return clk->map[sclk->id].set_rate(sclk, rate);
+	if (clk->map[sclk->id].set_rate)
+		return clk->map[sclk->id].set_rate(sclk, rate);
+
+	return -ENOTSUPP;
 }
 
 static int hsdk_cgu_disable(struct clk *sclk)