feat(st-clock): stub fdt_get_rcc_secure_state

The function fdt_get_rcc_secure_state() is only used in BL32.
Put it under flag to avoid compilation errors.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: If57c65bef0b1a3f7349527720f94fde26edbb73c
diff --git a/drivers/st/clk/stm32mp_clkfunc.c b/drivers/st/clk/stm32mp_clkfunc.c
index 3a63c90..379547f 100644
--- a/drivers/st/clk/stm32mp_clkfunc.c
+++ b/drivers/st/clk/stm32mp_clkfunc.c
@@ -248,6 +248,7 @@
 	return cuint;
 }
 
+#if defined(IMAGE_BL32)
 /*
  * Get the secure state for rcc node in device tree.
  * @return: true if rcc is configured for secure world access, false if not.
@@ -266,6 +267,7 @@
 
 	return true;
 }
+#endif
 
 /*
  * Get the clock ID of the given node in device tree.