tests: drivers: uart async test on the stm32l152re nucleo

Add configuration for testing the usart3 through the dma1
on the nucleo_l152re target.This test requires the Tx pin
to be connected to the RX pin on the board.
Pin definition is added for this usart instance.

Signed-off-by: Francois Ramu <francois.ramu@st.com>