stm32: stm32_min_dev.dtsi enable use of UART_2 and UART_3

For some reason UART_2 and UART_3 was not marked "okay" in the device
tree, making use difficult.
I have tested both UARTs on a "blue pill" board (stm32_min_dev_blue)
with both polling and RX/TX interrupts and found them to work as
expected.

Signed-off-by: Tommy Vestermark <tovsurf@vestermark.dk>