clk: clk_stm32mp1: Fix warnings when compiling with W=1

This patch solves the following warnings:

drivers/clk/clk_stm32mp1.c: In function 'stm32mp1_clk_get_parent':
warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i < ARRAY_SIZE(stm32mp1_clks); i++)
                ^
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/drivers/clk/clk_stm32mp1.c b/drivers/clk/clk_stm32mp1.c
index 5f15853..6ffa05b 100644
--- a/drivers/clk/clk_stm32mp1.c
+++ b/drivers/clk/clk_stm32mp1.c
@@ -805,10 +805,11 @@
 	const struct stm32mp1_clk_sel *sel = priv->data->sel;
 	int i;
 	int s, p;
+	unsigned int idx;
 
-	for (i = 0; i < ARRAY_SIZE(stm32mp1_clks); i++)
-		if (stm32mp1_clks[i][0] == id)
-			return stm32mp1_clks[i][1];
+	for (idx = 0; idx < ARRAY_SIZE(stm32mp1_clks); idx++)
+		if (stm32mp1_clks[idx][0] == id)
+			return stm32mp1_clks[idx][1];
 
 	i = stm32mp1_clk_get_id(priv, id);
 	if (i < 0)