Add pull-up to USART3 and UART7 RX pins to allow loop tests between USART3
and UART7 on stm32mp15 DKx boards.
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>