board: st: add missing gpio_request() for stm32f429-discovery

This fixes the following error message:

U-Boot 2019.07-rc4-00103-g5eea874b5e (Jun 25 2019 - 15:09:31 +0200)

DRAM:  8 MiB
gpio@40021800: dir_output: error: gpio GPIOG14 not reserved
gpio@40021800: dir_output: error: gpio GPIOG13 not reserved
gpio@40021800: set_value: error: gpio GPIOG14 not reserved
Flash: 2 MiB
....

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
diff --git a/board/st/stm32f429-discovery/led.c b/board/st/stm32f429-discovery/led.c
index 8320ad4..ae6df78 100644
--- a/board/st/stm32f429-discovery/led.c
+++ b/board/st/stm32f429-discovery/led.c
@@ -9,7 +9,9 @@
 
 void coloured_LED_init(void)
 {
+	gpio_request(CONFIG_RED_LED, "red led");
 	gpio_direction_output(CONFIG_RED_LED, 0);
+	gpio_request(CONFIG_GREEN_LED, "green led");
 	gpio_direction_output(CONFIG_GREEN_LED, 0);
 }