boards: reel_board: enable pull-up on UART TX pin

Enable pull-up on UART TX pin to reduce power consumption.
If the board is powered by battery the SoC consumes more
power than expected.
The consumption increases because TX pin is floating
(High-Impedance state of pin B from Dual-Supply Bus Transceiver).

Similar to commit b5b72849
("boards: reel_board: enable pull-up on UART RX pin")

Signed-off-by: Johann Fischer <j.fischer@phytec.de>