refactor(drivers/st/clk): change fdt_get_rcc_node as static

Change the fdt_get_rcc_node function to static, as it is used only in
stm32mp_clkfunc.c file; it is only a cleanup change without functional
modification.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Change-Id: Ib4ef110f6f1b16dbaa727a065e40275d3cf58a73
diff --git a/drivers/st/clk/stm32mp_clkfunc.c b/drivers/st/clk/stm32mp_clkfunc.c
index 8333f6d..2101171 100644
--- a/drivers/st/clk/stm32mp_clkfunc.c
+++ b/drivers/st/clk/stm32mp_clkfunc.c
@@ -161,7 +161,7 @@
  * @param fdt: Device tree reference
  * @return: Node offset or a negative value on error
  */
-int fdt_get_rcc_node(void *fdt)
+static int fdt_get_rcc_node(void *fdt)
 {
 	return fdt_node_offset_by_compatible(fdt, -1, DT_RCC_CLK_COMPAT);
 }