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

Enable pull-up on UART RX pin to reduce power consumption.
If the board is powered by battery and the debugger is
not connected via USB to the host, the SoC consumes up
to 2mA more than expected.
The consumption increases because RX pin is floating
(High-Impedance state of pin B from Dual-Supply Bus Transceiver).

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