ar71xx: fix Arduino Yun enabling of level shifters outputs

ar71xx: fix Arduino Yun enabling of level shifters outputs

As show in Arduino Yun schematic [1] GPIO 21 and 22 are connected to
output enable pin (OE) of two NTB01xx level shifters.

NTB01xx datasheets [2] [3] states that OE pin are active-high
therefore we should initialize GPIO 21 (DS_GPIO_OE) and GPIO 22
(DS_GPIO_OE2) accordingly to actually enable level shifters outputs.

[1] https://www.arduino.cc/en/uploads/Main/arduino-Yun-schematic.pdf
[2] https://www.nxp.com/docs/en/data-sheet/NTB0102.pdf
[3] https://www.nxp.com/docs/en/data-sheet/NTB0104.pdf

Signed-off-by: Edoardo Scaglia <edoardo.87@gmail.com>