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>