dst: gigadevice: change usart3 address to lower case
dst: gigadevice: change usart3 address to lower case

DTS node address should use lower case. This fix an incorrect
gd32f403 usart3 node address.

Signed-off-by: HaiLong Yang <c5b53c3148d79a31d58d6febcc7e03b084440d0f@brainco.cn>

finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr