boards/arm: stm32l562e_dk: Enabling usart3

This commit enables usart3 channel on stm32l562e_dk platform.
This would also be used to test usart in dma mode with
tests/drivers/uart/uart_async_api.

Signed-off-by: Krishna Mohan Dani <krishnamohan.d@hcl.com>
3 files changed